led flasher with normally closed switch

[jsn4728]

I am using a two transistor led flasher as part of an alarm in my RV. I need to be able to start the flasher by opening a normally closed switch. I have the led flasher working perfectly but I can't figure out how to switch it. I've tried adding another 3906 to switch current to the led but this just makes the led come on solid. I've tried several combinations of resistors with no luck. I'm powering the circuit with 12 volts. I have the complete circuit draw out how I think that it should work for reference. Could email if needed. Thanks.

 

[obg]

Assuming you have a ballast resistor for the LED you could simply connect your switch across the LED ie normally closed = shorted out, open switch = flashing LED

 

[jsn4728]

Thanks. That does work. So how does it work? When I short across the led does the current flow continue and does the pulsing continue?

 

[IRstuff]

Whatever is supplying the current to the LED is simply supplying it to the switch.

TTFN

 

[OperaHouse]

Add an extra NPN transistor. Connect the NC switch from base to emitter. Connect base to plus supply through 4.7K resistor. Depending on flasher draw and transistor gain, you could use a higher resistance. Connect emitter to minus supply. Connect collector to the old minus lead of the flasher circuit. When switch opens the transistor is turned on allowing current to flow to the flasher.

 

[nbucska]

you can get flashing LED from Radio Shack.

<nbucska@pcperipherals.com>