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I tried building the below schematic, but am having trouble. I've gone over it 3 times with no luck. I think there may be an error in the schematic. Could someone take a look?<p>Thanks
Terrapin<p>Headlight modulator schematics jpg

The oscillator appears to be drawn wrong. The feedback capacitor should connect directly from pin 10 to pins 12 and 13, while the 1.2M resistor should connect from pin 11 to pins 12 and 13. In other words, I think you need to omit the 1M resistor which connects to pins 12 and 13.

I hate to sound ignorant, but exactly what does a "headlight modulator" do? My first impressions are that it's illegal to use in a private vehicle as most state make flashing white lights on the front of a vehicle legal only for fire, police, EMT and coronors' vehicles .... even volunteer firefolk can't use them on their private vehicles; blue flashing lights only.<p>Dean

They are legal for motorcycles in all 50 states.<p>Mike

Right Mike. <p>The new DOT safety rules state that it is legal to use a headlight modulator on motorcycle in all 50 states. Of course there are some rules about the CPM and automatic switch stuff, but I wont bore you with that.<p>Terrapin

The oscillator circuit is OK, I have used that one quite a lot. The capacitor has to be low leakage, if you are using a Z5U ceramic, it may not work. Use a mylar or poly cap.

The fact that the cap is ceramic or Z5U shouldn't affect things other than frequency. The Z5U ceramic isn't a leaky cap compared to polyester ... it just has a nasty tempco. And the danged thing changes capacitance value depending upon the dc voltage applied.<p>Dean

I don't know if the oscillator is right, but the schematic shows a pair of N channel MOSFETS, in parallel. As I recall the Drain has to be positive and the source Negative. It appears as the circuit is to essentially turn on the lamp or turn off the lamp the circuit is in series with.
The way I see it the MOSFETS will never turn on.<p>Ned

it seems to me that if the oscilator is working you need to get the headlamp connected between the drain of the mosfet ant the positave sopply for the headlamp.

Ned is correct, however the symbol for the MOSFET is drawn OK, it is the S & D that are swapped.

Ok, 2 things.<p>1) I need to add a second ground to the mosfet? Because It will be in series with bulb on the hot side.<p>2) I also need to swith the the D & S on the mosfet?<p>Thanks,
Terrapin<p>P.s. If someone would like to come up with a better plan I'm all ears, one thing though there needs to be 240cpm to be legal.<p>Thanks again.

If you put it on the hot side, after switching the drain and source, hook the positive side to 12v and the negative side to the llght. I think that should work since power to the control circuit is bootstrapped, meaning it is referenced to the source of the mosfet. Every time the mosfet turns off, the relatively low impedance of the headlight charges up the 100uF capacitor through the diode. When the mosfet turns back on, the voltage on the control circuit is pulled up by the source of the mosfet and the diode is reversed biased so it does not discharge the 100uF capacitor.<p>I still don't like the oscillator passive component values. The resistors are a bit high for reliable switching of the MAND gate inputs especially if they get warm and leak more. If you have an oscilloscope, breadboard out the circuit and try lowering the resistor values by a factor of 10 and increasing the capacitor to 1uF on the top oscillator. I recommend an oscilloscope because you said that the frequency has to be exact. The capacitors should be bi-polar types.
The purpose of the original 1meg Ohm reistors on some of the gates as far as I see it is to protect the gate inputs from voltage spikes. If you decide to keep them, lower their value to 10k Ohm. Am I off base here people? Maybe just try out the circuit by using the information in the first paragraph of this post and see how it works, going on with the info in this paragraph if it still doesn't work right.