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Electronic 12 Volt Emergency Flasher


Active Member
Thanks "Diver300" I understand. That is it though the led's see some voltage coming out of the 49a output of the flasher when the key is on but no turnsignals are in use. The indicators are directly connected to 49a. I will have to add a resistor across my leds or across 31 to 49a on the flasher probably a 2K ohm would stop the glow. It's just an artifact of the new flasher that the old one did not have.
I found a couple of other flasher threads that don't seem to go anywhere in the end as far as a viable solid state relay flasher design .


Active Member
My plan as far as coming up with a 12 Volt Electronic 3 pin flasher is to use a Timer chip like the ICM7555 and the IRF-540 Power MOSFET and power the circuit at 20 volts using a small Boot Converter circuit board that I can buy on e-bay. This will allow the N channel Power MOSFET to operate with the load on its Source. I will work on a schematic. Any thoughts?
Here: Also ordered INT ICM755IPA Datasheet.
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Active Member
I just realized I may not need the DC to DC converter to raise the voltage. Here is what I am now thinking. Just a capacitor resistor and diode and I get a voltage doubling to 24 volts for the gate circuit. Will it work?
IRL540N Gatting.JPG


Active Member
Exciting Update!
The 7555 chips arrived and they work quite well at controlling the flash rate:

This Working Design overcomes the problem of fully gating the IRF-540 MOSFET with the Load on its "Source" by using a Voltage Doubler Circuit to raise the Gate Power Supply to above 22.5 volts dc. The 2N2222 transistor inverts the signal however with a signal frequency of 1 second and a 50% duty cycle that does. not matter.
For some reason when I lowered the power supply by turning it down slowly the flasher works fine to about 7 volts then it sticks on blowing the 7555 chip. Tested fine though powering it up between 10.5 and 15 volts DC supply.

Sch Flasher 7555 .JPG

Gate Power Supply voltage across the 100 uf cap with 12 volt main supply and 5 volts/Div with Zero 2 div below center 17-22.5 volts total:

Output at load Rise and Fall (Note main supply voltage 10.5 VDC and Scope set to 5V/Div) (Zero volt line is 2 div below center line)
Output Rise.jpgOutput at Load Fall.jpg

Gate ON Pulse Voltage seems to always be 22.5. VDC
Gate Pulse 22.5 volts.jpg

7555 Flasher Prototype:
Flasher Working.jpg
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Active Member
Installed it in the car and it worked till I started the engine etc... Burned out another 7555 chip! Made some revisions and it seems to stay working now. It's a little fussy at Idle where the Alternator Voltage is not stable. It flashes 3 normal flashers then 3 quicker flashes but just at idle. It probably needs to have the 7555 regulated down to 10.5 volts or so then it would be stable I think. This design seems to flash at a steady rate with various loads the exception being when the idle voltage is not stable between 12-14 vdc. (Note: The 540 MOSFET stays cool on just 2 turn signals for a load but starts to get Hot With 3 lamps. One of my E-Flasher Lamps is not working not sure it could handle all four 4 lamps without a Heatsink.)
Sch Flasher 7555 0001.JPG


Active Member
Ok, So Now there is a 2nd Design not a whole lot different it uses a different MOSFET only 2.5m Ohms it's a "Power Trench" Mosfet. Also instead of all the diode protection there is now a LM7805 Regulator running the ICM 7555 Timer Chip. Also I constructed this flasher to fit into a old flasher relay housing. Did some testing it's seeing about 19 Volts, a nice Square Wave at the MOSFET gate and its timing a lot more stable than the 1st design did installed in the car. Here is the Schematic "2nd Design" and a Gate Signal Scope photo and some photos of the prototype. This seems to be a better design, It should handle all 4 Lamps ok.

(The only artifacts I see during testing with it is it continues to operate all the time even when not loaded, a 15K ohm resistor across the MOSFET is enough to balance my LED turnsignal indicators to stop them flashing. It would probably flash incandescent indicators continuously though!)

Sch Flasher 7555 2nd.JPG

Here the 19 volt Gate signal with the scope at 5 volts/Div and the Time base on .2 seconds however it wasn't set to cal on the Vernier but the ICM 7555 cycle is about 1 second:
2nd Design Flasher.jpg2nd Flasher 2.jpg2nd Flasher 1.jpg

2nd Flasher 3.jpg

In this photo the Flasher Cycle Time Resistors are shown:
2nd Flasher 6.jpg
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Active Member
Well The Turn Signal Flasher has failed, It lasted 10 or 11 months, not sure why, I will investigate. Of note I replaced it with the replacement mechanical and the lights work fine so the problem is in the flasher circuit.


Well-Known Member
Most Helpful Member
For your interest, here's the LTspice simulation of a fairly simple two-terminal flasher circuit that uses one CMOS CD4093 Schmidt-trigger quad NAND gate package and two MOSFETs plus a few passive parts, to perform the flasher function.
C2 is charged through the synchronous rectifier M2 when M1 is off.
(A synchronous rectifier is used since it also discharges C2 at the end of one turn cycle, so it starts immediately at the start of the next cycle).
The C2 voltage is then bootstrapped to provide power to the circuit when the MOSFET turns on (which requires very little power due to the CMOS circuit) so the circuit can drive a grounded load.
U1d provides a short delay to allow the bootstrap to charge before the circuit starts flashing.
The other three NAND gates are configured as an Astable multivibrator.
M2 can be just about any logic-level P-MOSFET (Maximum Vgs(th) <2V).

I believe it can substitute for most 3-terminal flashers as well.

(Edit: Synchronous rectifier substituted for diode.)

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