Led Circuits Page

1.5 Volt LED Flashers

The LED flasher circuits below operate on a single 1.5 volt battery. The circuit on the upper right uses thepopular LM3909 LED flasher IC and requires only a timing capacitor and LED.

The top left circuit, designed by Andre De-Guerin illustrates using a 100uF capacitor to double the batteryvoltage to obtain 3 volts for the LED. Two sections of a 74HC04 hex inverter are used as a squarewaveoscillator that establishes the flash rate while a third section is used as a buffer that charges the capacitorin series with a 470 ohm resistor while the buffer output is at +1.5 volts. When the buffer output switchesto ground (zero volts) the charged capacitor is placed in series with the LED and the battery which suppliesenough voltage to illuminate the LED. The LED current is approximately 3 mA, so a high brightness LED isrecommended.

In the other two circuits, the same voltage doubling principle is used with the addition of a transistor toallow the capacitor to discharge faster and supply a greater current (about 40 mA peak). A larger capacitor(1000uF) in series with a 33 ohm resistor would increase the flash duration to about 50mS. The discrete 3transistor circuit at the lower right would need a resistor (about 5K) in series with the 1uF capacitor towiden the pulse width.Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

AC Line powered LEDs

The circuit below illustrats powering a LED (or two) from the 120 volt AC line using a capacitor to drop thevoltage and a small resistor to limit the inrush current. Since the capacitor must pass current in bothdirections, a small diode is connected in parallel with the LED to provide a path for the negative half cycleand also to limit the reverse voltage across the LED. A second LED with the polarity reversed may besubsituted for the diode, or a tri-color LED could be used which would appear orange with alternatingcurrent. The circuit is fairly efficient and draws only about a half watt from the line. The resistor value (1K/ half watt) was chosen to limit the worst case inrush current to about 150 mA which will drop to less than30 mA in a millisecond as the capacitor charges. This appears be a safe value, I have switched the circuiton and off many times without damage to the LED. The 0.47 uF capacitor has a reactance of 5600 ohms at60 cycles so the LED current is about 20 mA half wave, or 10 mA average. A larger capacitor will increasethe current and a smaller one will reduce it. The capacitor must be a non-polarized type with a voltagerating of 200 volts or more.

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The lower circuit is an example of obtaining a low regulated voltage from the AC line. The zener diodeserves as a regulator and also provides a path for the negative half cycle current when it conducts in theforward direction. In this example the output voltage is about 5 volts and will provide over 30 milliampswith about 300 millivolts of ripple. Use caution when operating any circuits connected directly to the ACline.Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

LED Traffic Lights

The LED traffic Light circuit controls 6 LEDs (red, yellow and green) for both north/south directions andeast/west directions. The timing sequence is generated using a CMOS 4017 decade counter and a 555timer. Counter outputs 1 through 4 are wire ORed using 4 diodes so that the (Red - North/South) and(Green - East/West) LEDs will be on during the first four counts. The fifth count (pin 10) illuminates (Yellow- East/West) and (Red - North/South). Counts 6 through 9 are also wire ORed using diodes to control (Red- East/West) and (Green - North/South). Count 10 (pin 11) controls (Red - East/West) and (Yellow -North/South). The time period for the red and green lamps will be 4 times longer than for the yellow andthe complete cycle time can be adjusted with the 47K resistor. The eight 1N914 diodes could be subsitutedwith a dual 4 input OR gate (CD4072).Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

40 LED Bicycle Light

The 555 circuit below is aflashing bicycle lightpowered with four C,D orAA cells (6 volts). Two setsof 20 LEDs will alternatelyflash at approximately 4.7cycles per second using RCvalues shown (4.7K for R1,150K for R2 and a 1uFcapacitor). Time intervalsfor the two lamps areabout 107 milliseconds(T1, upper LEDs) and 104milliseconds (T2 lowerLEDs). Two transistors areused to provide additionalcurrent beyond the 200



mA limit of the 555 timer.A single LED is placed in

series with the base of the PNP transistor so that the lower 20 LEDs turn off when the 555 output goeshigh during the T1 time interval. The high output level of the 555 timer is 1.7 volts less than the supplyvoltage. Adding the LED increases the forward voltage required for the PNP transistor to about 2.7 volts sothat the 1.7 volt difference from supply to the output is insufficient to turn on the transistor. Each LED issupplied with about 20 mA of current for a total of 220 mA. The circuit should work with additional LEDs upto about 40 for each group, or 81 total. The circuit will also work with fewer LEDs so it could be assembledand tested with just 5 LEDs (two groups of two plus one) before adding the others.Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

16 Stage Bi-Directional LED Sequencer

The bi-directional sequencer uses a 4 bit binary up/down counter (CD4516) and two "1 of 8 line decoders"(74HC138 or74HCT138) togenerate thepopular "NightRider" display.A SchmittTriggeroscillatorprovides theclock signalfor thecounter andthe rate canbe adjustedwith the 500Kpot. TwoadditionalSchmittTrigger

inverters are used as a SET/RESET latch to control the counting direction (up or down). Be sure to use the74HC14 and not the 74HCT14, the 74HCT14 may not work due to the low TTL input trigger level. When thehighest count is reached (1111) the low output at pin 7 sets the latch so that the UP/DOWN input to thecounter goes low and causes the counter to begin decrementing. When the lowest count is reached (0000)the latch is reset (high) so that the counter will begin incrementing on the next rising clock edge. Thethree lowest counter bits (Q0, Q1, Q2) are connected to both decoders in parallel and the highest bit Q3 isused to select the appropriate decoder. The circuit can be used to drive 12 volt/25 watt lamps with theaddition of two transistors per lamp as shown below in the section below titled "Interfacing 5 volt CMOS to12 volt loads".Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

Expandable 16 Stage LED Sequencer

The circuit below uses a hex Schmitt Trigger inverter (74HC14) and two 8 bit Serial-In/Parallel-Out shiftregisters(74HCT164 or74HC164) tosequence 16LEDs. Thecircuit can beexpanded togreater lengthsby cascadingadditional shiftregisters andconnecting the8th output (pin13) to the datainput (pin 1) ofthe succeedingstage. A Schmitttrigger oscillator(74HC14 pin 1 and 2) produces the clock signal for the shift registers, the rate being approximately 1/RC.Two additional Schmitt Trigger stages are used to reset and load the registers when power is turned on.Timing is not critical, however the output at pin 8 of the Schmitt Trigger must remain high during the firstLOW to HIGH clock transition at pin 8 of the registers, and must return low before the second rising edgeto load a single bit. If the clock rate is increased, the length of the signal at pin 9 of the Schmitt Triggershould be reduced proportionally to avoid loading more than one bit. The HCT devices will normally provideabout 4 mA (source or sink) from each output but can supply greater currents (possibly 25 mA) if only oneoutput is loaded. The common 150 ohm resistor restricts the current below 25 mA using a 6 volt powersource. If the circuit is operated with two or more LEDs on at the same time, resistors may be needed inseries with each LED to avoid exceeding the maximum total output current for each IC of 25 mA. Forgreater brightness, individual buffer transistors can be used as shown in the 10 stage LED sequencer onthis same page.Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

10 Channel LED Sequencer

The 4017 is a CMOS decade counter with 10 decoded outputs. Inputs include a CLOCK (pin14), a RESET(PIN15), and a CLOCK INHIBIT (pin 13). The clock input drives an internal schmitt trigger circuit for pulseshaping and allows for unlimited clock rise and fall times. The counter is advanced one count at the risingedge of the clock signal if the CLOCK INHIBIT line is low. A high RESET signal resets the counter to thezero count. The circuit may be configured for counts less than 10 by connecting RESET to an output pinone above the desired count. Thus, a five channel sequencer could be made by connecting pin 15 to pin 1.A CARRY-OUT signal (pin 12) may be used to clock subsequent stages in a multi-device counting chain(ones, tens, hundred, etc). Small signal NPN transistors are used to increase the output current for theLEDs to about 20ma which is set by the common 120 ohm resistor. Other NPN transistors may besubstituted for the 3904. The 555 timer generates the clock signal, the frequency being determined by the1uF capacitor and 47K resistor which is approximately = 1.44 / 2RC = 15 Hz.Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

Two Transistor LED Flasher

This circuit will flash a bright red LED (5000 mcd) as an attentiongetting device or fake car alarm. Component values are not criticaland other transistors may be used. Flash duration is determined byR2 and C1 and is approximately 3 time constants (3*R2*C1).Brightness is controlled by R3 wich limits the LED current to about 20mA for values listed. R1 provides bias for the transistors which shouldbe low enough not to saturate Q2 with the capacitor disconnected. Ifthe circuit does not oscillate, R1 may be too low or R2 may be toohigh. D1 allows for higher duty cycle operation and limits the reversevoltage at the base of Q1 to -0.7 V. D1 may be omitted for low voltage(3-9) and low duty cycle operation. Most parts available at RadioShack.Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

Volts R1 R2 R3 C1 Approx. Flash Rate12 10 Mohm 22 Kohm 470 ohm 0.47 uF 140 per minute12 10 Mohm 10 Kohm 470 ohm 1 uF 60 per minute9 6.8 Mohm 1 Kohm 390 ohm 6.8 uF 15 per minute6 3.3 Mohm 10 Kohm 220 ohm 1 uF 80 per minute3 1.5 Mohm 10 Kohm 51 ohm 1 uF 120 per minute3 3.3 Mohm 47 Kohm 51 ohm 0.47 uF 140 per minute

Fading Red Eyes

This circuit can be used to slowly illuminate and fade a pair of red LEDs. A linear 3 volt p-p rampingwavform is generated at pin 1 of the IC andbuffered with an emitter follower transistorstage. The 22uF capacitor and 47K resistorconnected to pin 2 establish the frequencywhich is about 0.5 Hz.Original scheme edited by Bill Bowden,http://www.bowdenshobbycircuits.info

28 LED Clock Timer

Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

This is a programmable clock timer circuit that uses individual LEDs to indicate hours and minutes. 12 LEDscan be arranged in a circle to represent the 12 hours of a clock face and an additional 12 LEDs can bearranged in an outer circle to indicate 5 minute intervals within the hour. 4 additional LEDs are used toindicate 1 to 4 minutes of time within each 5 minute interval.

The circuit is powered from a small 12.6 volt center tapped line transformer and the 60 cycle linefrequency is used for the time base. The transformer is connected in a full wave, center tappedconfiguration which produces about 8.5 volts unregulated DC. A 47 ohm resistor and 5.1 volt, 1 watt zenerregulate the supply for the 74HCT circuits.

A 14 stage 74HCT4020 binary counter and two NAND gates are used to divide the line frequency by 3600producing a one minute pulse which is used to reset the counter and advance the 4017 decade counter.The decade counter counts the minutes from 0 to 4 and resets on the fifth count or every 5 minutes whichadvances one section of a dual 4 bit binary counter (74HCT393). The 4 bits of this counter are thendecoded into one of 12 outputs by two 74HCT138 (3 line to 8 line) decoder circuits. The most significant bitis used in conjunction with an inverter to select the appropriate decoder. During the first eight counts, thelow state of the MSB is inverted to supply a high level to enable the decoder that drives the first 8 LEDs.During counts 9 to 12, the MSB will be high and will select the decoder that drives the remaining 4 LEDswhile disabling the other decoder. The decoded outputs are low when selected and the 12 LEDs areconnected common anode with a 330 ohm current limiting resistor to the +5 volt supply. The 5th output ofthe second decoder (pin 11) is used to reset the binary counter so that it counts to 11 and then resets tozero on the 12th count. A high reset level is required for the 393 counters, so the low output from the lastdecoder stage (pin 11) is inverted with one section of a 74HCT14 hex Schmitt trigger inverter circuit. A10K resistor and 0.1uF cap are used to extend the reset time, ensuring the counter receives a reset signalwhich is much longer than the minimum time required. The reset signal is also connected to the clock input(pin 13) of the second 4 bit counter (1/2 74HCT393) which advances the hour LEDs and resets on the 12thhour in a similar manner.

Setting the correct time is accomplished with two manual push buttons which feed the Q4 stage (pin 7) ofthe 4020 counter to the minute and hour reset circuits which advance the counters at 3.75 counts persecond. A slower rate can be obtained by using the Q5 or Q6 stages. For test purposes, you can use Q1(pin 9) which will advance the minutes at 30 per second.

The time interval circuit (shown below the clock) consists of a SET/RESET flipflop made from the tworemaining NAND gates (74HCT00). The desired time interval is programmed by connecting the anodes ofthe six diodes labeled start, stop and AM/PM to the appropriate decoder outputs. For example, to turn therelay on at 7:05AM and turn it off at 8:05AM, you would connect one of the diodes from the start sectionto the cathode of the LED that represents 7 hours, the second diode to the LED cathode that represents 5minutes and the third diode to the AM line of the CD4013. The stop time is programmed in the samemanner. Two additional push buttons are used to manually open and close the relay. The low start and stopsignals at the common cathode connections are capacitively coupled to the NAND gates so that the manualpush buttons can override the 5 minute time duration. That way, you can immediately reset the relaywithout waiting 5 minutes for the start signal to go away.

The two power supply rectifier diodes are 1N400X variety and the switching diodes are 1N914 or 4148s butany general purpose diodes can be used. 0.1 uF caps (not shown on schematic) may be needed near thepower pins of each IC. All parts should be available from Radio Shack with the exception of the 74HCT4017decade counter which I didn't see listed. You can use either 74HC or 74HCT parts, the only differencebetween the two is that the input switching levels of the HCT devices are compatible with worst case TTLlogic outputs. The HC device inputs are set at 50% of Vcc, so they may not work when driven frommarginal TTL logic outputs. You can use a regular 4017 in place of the 74HCT4017 but the output currentwill much lower (less than 1 mA) and 4 additional transistors will be required to drive the LEDs. Withoutthe buffer transistors, you can use a 10K resistor in place of the 330 and the LEDs will be visible, but very

dim. Using the 4017 to drive LEDs with transistor buffers is shown in the "10 Channel LED Sequencer" atthe top of this page.

72 LED Clock


In the circuit below, 60 individual LEDs are used to indicate the minutes of a clock and 12 LEDs indicatehours. The power supply and time base circuitry is the same as described in the 28 LED clock circuit above.The minutes section of the clock is comprised of eight 74HCT164 shift registers cascaded so that a singlebit can be recirculated through the 60 stages indicating the appropriate minute of the hour. Only two of theminutes shift registers are shown connected to 16 LEDs. Pin 13 of each register connects to pin 1 of thenext for 7 registers. Pin 6 of the 8th register should connect back to pin 1 of the first register using the47K resistor. Pins 2,9,8, 14 and 7 of all 8 minutes registers (74HC164) should be connected in parallel (pin8 to pin 8, pin 9 to pin 9, etc.). The hours section contains two 8 bit shift registers and works the same wayas the minutes to display 1 of 12 hours. Pin 9 of all 74HCT164s (hours and minutes) should be connectedtogether. For 50 Hertz operation, the time base section of the circuit can be modified as shown in the lowerdrawing labeled "50 Hertz LED Clock Time Base". You will need an extra IC (74HC30) to do this since itrequires decoding 7 bits of the counter instead of 4. The two dual input NAND gates (1/2 74HC00) that arenot used in the 50 Hertz modification should have their inputs connected to ground.

When power is applied, a single "1" bit is loaded into the first stage of both the minutes and hoursregisters. To accomplish this, a momentary low reset signal is sent to all the registers (at pin 9) and also aNAND gate to lock out any clock transitions at pin 8 of the minutes registers. At the same time, a highlevel is applied to the data input lines of both minutes and hours registers at pin 1. A single positive goingclock pulse (at pin 8) is generated at the end of the reset signal which loads a high level into the first stageof the minutes register. The rising edge of first stage output at pin 3 advances the hours (at pin 8) and asingle bit is also loaded into the hours register. Power should remain off for about 3 seconds or more beforebeing re-applied to allow the filter and timing capacitors to discharge. A 1K bleeder resistor is used acrossthe 1000uF filter capacitor to discharge it in about 3 seconds. The timing diagram illustrates the power-onsequence where T1 is the time power is applied and beginning of the reset signal, T2 is the end of the resetsignal, T3 is the clock signal to move a high level at pin 1 into the first register, T4 is the end of the datasignal. The time delay from T2 to T3 is exaggerated in the drawing and is actually a very short time of justthe propagation delay through the inverter and gate.

Two momentary push buttons can be used to set the correct time. The button labeled "M" will incrementthe minutes slowly and the one labled "H" much faster so that the hours increment slowly. The hoursshould be set first, followed by minutes.

50 Hertz LED Clock Timebase


Infrared Remote Control Tester

This is a fairly easy circuit that canbe used to test TV and VCR remotecontrols. The infrared detectormodule (GP1U52X) (Radio Shack276-137) produces a 5 volt TTLpulse train corresponding to thedigital code of the particular remotecontrol key pressed. In the lowercircuit, the module output isnormally low with no signalreceived and becomes a positivegoing pulse train when a signal ispresent. Other detector modulesare available that have an invertedoutput as shown in the upperdrawing which is the type I used,but I don't have the part number, Ibelieve it was removed from a VCR.The pulse sequence represents thedigital code of the particular keypressed along with possible

manufacturer information. As the pulse train occurs, the 4.7uF capacitor is charged to about 3 volts andthe capacitor voltage minus a diode drop appears across the 470 ohm resistor yielding a collector currentfrom the 2N3904 or 2N3906 of about 5 milliamps. The collector current of the first stage flows into thebase of the output transistor (MJE34 or 2N3053) which delivers around 250 mA into the indicator lamps.When the pulse train ends, the capacitor slowly discharges through the base of the first stage transistorallowing the Xmas tree lights to remain on for a about 1 second. The little Xmas lamps will operate over awide voltage range, so you can use bulbs from almost any string, but bulbs from shorter strings (35 orless) will probably last longer operated at 5 volts.

The circuit can be powered from a small 9-12 volt DC, 250 mA or greater wall transformer. It may alsoneed an additional 1000 uF filter capacitor across the DC output if the wall transformer does not have abuilt in capacitor. For use with a 9 volt battery, the incandescent lamps can be replaced with a regular LEDand 680 ohm resistor and the output transistors can be replaced with small signal transistors (2N3904 or2N3906). The total current drain will be about 25 mA with the LED lit, and 15 mA standby when the LED isoff.Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

Battery Equal Charge Indicator

The circuit below illuminates an LED to indicate unequal charges between two 12 volt lead batteries. It canbe used to verify that two batteries areconnected in parallel or isolated sincethe LED will be off when the voltagesare equal within a tollerance, or on ifthe voltage difference is greater than100 millivolts. Three comparators andthree voltage dividers are used todetermine battery conditions. The upperleft comparator (+) input at pin 5 is setto about 10 volts with battery #1 at 12volts. The negative input (pin 4) is setto a slightly lower voltage by adding anadditional 240 ohms to the voltagedivider so that the output of thecomparator will be positive when bothbattery voltages are equal and negativeif battery 2 rises above battery 1 by 100millivolts or more. The voltage at pin 5is used as a reference for the lowercomparator and the negative input ofthe lower comparator is set to a lower voltage with the addition of 510 ohms, so that the output will also

be positive when the battery voltages are equal and negative when battery #1 is greater than #2 by 100millivolts or more. The two comparator outputs are both connected to the positive input of the thirdcomparator at pin 9 so that the LED will illuminate when either condition exists,(Battery #1 > Battery #2) OR (Battery #2 > Battery #1).Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

Astable Multivibrator

This is an astable multivibrator circuit to alternately flash two LEDs. TheR and C values determine the frequency and the 470ohm collector loadresistor set the current to about 20mA when the circuit is operated at 12V. Frequency is about 1 cycle per second using 22uF capacitors and 47Kresistors. Smaller R or C values will increase frequency. The LEDs mayalso be vired in series with the collector resistors and other transistorscan be used. Reverse the supply voltage connections and LEDconnections if PNP type transistors are used.Original scheme edited by Bill Bowden, http://www.bowdenshobbycircuits.info

LED Photo Sensor.

Here's a circuit that takes advantage of the photo-voltaic voltage of an ordinary LED. The LED voltage isbuffered by a junction FET transistor and then applied to the inverting input of an op-amp with a gain ofabout 20. This produces a change of about 5 volts at the output from darkness to bright light. The 100Kpotentiometer can be set so that the output is around 7 volts in darkness and falls to about 2 volts inbright light.


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