{"id":1438,"date":"2014-03-15T22:14:48","date_gmt":"2014-03-16T02:14:48","guid":{"rendered":"http:\/\/hackaweek.com\/hacks\/?p=1438"},"modified":"2014-03-17T07:48:47","modified_gmt":"2014-03-17T11:48:47","slug":"diy-emergency-vehicle-flasher-part-3","status":"publish","type":"post","link":"http:\/\/hackaweek.com\/hacks\/diy-emergency-vehicle-flasher-part-3\/","title":{"rendered":"DIY Emergency Vehicle Flasher Part 3"},"content":{"rendered":"

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. <\/p>\n

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. <\/p>\n

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<\/del> 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<\/del> 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.<\/p>\n

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.<\/p>\n

Below you’ll find a parts list, link to parts suppliers, a PCB layout, the schematic and the final build video.<\/p>\n

UPDATE:<\/strong> See Timo’s comment below for an alternative circuit. <\/p>\n

Keep on hackin!<\/p>\n

Parts list:
\n2 – 20 ohm 1\/4 watt resistors – R8, R9
\n7 – 1K 1\/4 watt resistors – R1, R3, R4, R6, R10, R11, R13
\n2 – 3K 1\/4 watt resistors – R5, R7
\n1 – 20K 1\/4 watt resistor – R12
\n1 – 20K potentiometer – R2
\n3 – 0.1 uF ceramic capacitors – C1, C3, C5
\n2 – 1 uF 25V electrolytic capacitors – C4, C6
\n1 – 100 uF 25V electrolytic capacitor – C2
\n2 – N-channel MOSFET transistors – Q1, Q2
\n1 – P-channel MOSFET transistor – Q3
\n1 – 1N4004 rectifier diode – D1
\n1 – 1N4742 xener diode – D2
\n3 – NE555 timer dip chips – IC1, IC2, IC3
\n1 – SPST switch – SW1
\n1 – Knob for the potentiometer
\n1 – Perfboard or PCB
\n2 – LED light arrays
\n1 – Project enclosure
\nSeveral feet of 16 gauge two strand wire<\/strong><\/p>\n

Parts suppliers<\/strong>
\nTayda Electronics<\/a>
\nAllied Electronics<\/a>
\nDigiKey<\/a>
\nRadio Shack<\/a><\/p>\n

PCB Layout<\/strong>
\n\"Adjustable<\/a><\/p>\n

ExpressPCB file<\/strong>
\nflasher pcb layout.pcb<\/a><\/p>\n

Schematic<\/strong>
\n\"Adjustable<\/a><\/p>\n

\"diy<\/a><\/p>\n