DIY Emergency Vehicle Flasher Part 3

I left off last week with a few problems. The second LED array would not go all the way off and after some troubleshooting this week I realized that the circuit was not working properly due to a faulty n-channel MOSFET. This was probably caused by the odd way I was powering up the third 555 in the circuit.

After abandoning the idea of using a small switching transistor to turn on the third 555 an idea hit me. Why not use a p-channel MOSFET? When pin 3 of the first 555 goes low I could utilize the ground signal from that pin to pull the gate of the p-channel MOSFET (Q3) to ground thus turning it on which would allow voltage to flow from the positive rail through the MOSFET to pin 8 of the third 555. I tested it out first on a breadboard and it worked fine so I added it to the schematic and installed it on the perfboard.

The operation of the circuit is as follows. All three 555s are configured as astable oscillators. The first 555 is set to cycle high for .1 to 1 second depending on the position of R2 potentiometer. The positive output from pin 3 goes to the second 555 which powers it up. It then oscillates at approximately 40 21 hz on pin 3 and feeds a positive voltage to the gate of the n-channel MOSFET (Q1) which turns it on and off rapidly thus supplying a ground to the first LED array. When pin 3 goes low on the first 555 the ground signal pulls the gate of the p-channel MOSFET (Q3) to ground and turns it on which feeds a positive voltage to the third 555. This 555 then oscillates a 40 21 hz on pin 3 and feeds a positive voltage to the gate of the second n-channel MOSFET (Q2) which turns it on and off rapidly thus supplying a ground to a second LED array. The result is an alternate rapid flashing of two LED arrays with adjustability in the rate via the R2 20K potentiometer.

This circuit could also drive other lights providing the load does not exceed the amperage rating of the n-channel MOSFETS. It could also be modified to run other devices such as relays or stepper motors. Of course, all of this could be done with a micro controller but I chose the 555 because the design and build of the circuit is a good teaching tool for those wanting to learn beginning electronics.

Below you’ll find a parts list, link to parts suppliers, a PCB layout, the schematic and the final build video.

UPDATE: See Timo’s comment below for an alternative circuit.

Keep on hackin!

Parts list:
2 – 20 ohm 1/4 watt resistors – R8, R9
7 – 1K 1/4 watt resistors – R1, R3, R4, R6, R10, R11, R13
2 – 3K 1/4 watt resistors – R5, R7
1 – 20K 1/4 watt resistor – R12
1 – 20K potentiometer – R2
3 – 0.1 uF ceramic capacitors – C1, C3, C5
2 – 1 uF 25V electrolytic capacitors – C4, C6
1 – 100 uF 25V electrolytic capacitor – C2
2 – N-channel MOSFET transistors – Q1, Q2
1 – P-channel MOSFET transistor – Q3
1 – 1N4004 rectifier diode – D1
1 – 1N4742 xener diode – D2
3 – NE555 timer dip chips – IC1, IC2, IC3
1 – SPST switch – SW1
1 – Knob for the potentiometer
1 – Perfboard or PCB
2 – LED light arrays
1 – Project enclosure
Several feet of 16 gauge two strand wire