Just to throw another monostable/power-on-reset circuit option into the mix.

Ken

Simplicity. How is the 1s calculated, or is it a "Close to 1 Second w/2N7000"?

 

I suppose it's related to Vgs vs. Vds of the 2N7000 and the drop-out voltage of the relay...but, really it was just trial and error for a non-critical timing circuit.


And there's a OOPS! in my schematic. D1 should be across R1, not C1. It's there to speed up the reset discharge. I'll go back and correct it.

Ken

 

Nice one too.... but how can i make it for positive output?

 

Connect a +12V line to the relay's common (middle) switch contact and the "NO" (normally open) contact will be your 1-second +12v-pulsed output connection.

Do you know how much current is required from the +12v pulse to switch-on the defroster circuit? I assume you already know where to connect the +12v-pulsed line to the existing wiring. If you have an ammeter, connect the positive lead to +12v and touch the negative lead to the defroster's 'switched' connection. It could read anywhere from hundreds of mA to a few uA. If it's under a few hundred mA, I go with Bill's 555 circuit. But, remove K1, CR1, and T1. Replace R2 with CR1, with the anode to pin #3. The cathode will be your pulsed +12V signal. The diode isolates the circuit from manual operation of the defroster switch. It will be smaller and require less power without the relays.

Ken

 

Great job guys. In Multisim is working. I will make the real pcb and waiting for some sunshine to come....is very cold outside to wire the circuit in the car

 

Great job guys. In Multisim is working. I will make the real pcb and waiting for some sunshine to come....is very cold outside to wire the circuit in the car

On that note, be sure to order automotive rated caps for any electrolytics, they are a bit more expensive, but worth it. Cars are about the harshest environment one can put electronics into, next to space.

 

I posted this in a different thread... have a look at these:
http://parametric.linear.com/html/timerblox

Mike T.

 

I posted this in a different thread... have a look at these:
http://parametric.linear.com/html/timerblox

Mike T.

Are you getting kickbacks from Linear. I've seen this link 3 times today! He He.

iONic

 

Are you getting kickbacks from Linear. I've seen this link 3 times today! He He.

iONic

No, just trying to be helpful. Lot's of people looking at 555 circuits, these are simpler. Did you know about them before I posted the link?

Mike T.

 

No, just trying to be helpful. Lot's of people looking at 555 circuits, these are simpler. Did you know about them before I posted the link?

Mike T.

Mike,

A big issue is cost and availability for a one-off project. Digikey has them for ~$3 each in single quantities. Also, the packages are only in surface mount, the voltage is 2-5V, the output is 20mA... There are definitely places for them, but for general hobby projects, they have many limitations. I sort agree with iONIc, in that I think it is best to suggest them for projects where their packages and parameters shine, not just an unqualified replacement for 555s.

Ken

 

Mike,

A big issue is cost and availability for a one-off project. Digikey has them for ~$3 each in single quantities. Also, the packages are only in surface mount, the voltage is 2-5V, the output is 20mA... There are definitely places for them, but for general hobby projects, they have many limitations. I sort agree with iONIc, in that I think it is best to suggest them for projects where their packages and parameters shine, not just an unqualified replacement for 555s.

Ken

OFF TOPIC: Pardon me. I didn't see anything about being this being a one-off project. Nor that it had to be through-hole, reflect a certain price range or any particular voltage. From my perspective, one-off projects are not normally subject to a price cap. 2-5 volts; a resistor and a zener. Output 20mA; more than enough to turn on a FET. SMT; I think I saw in the thread something about being "smaller and less power than using a relay". I see question after question answered with "555" and all the convolusions gone through to make them work. I suggested an alternative, that's all. If you had never wandered past the 555 you are now aware of them. "Unqualified replacement"? I don't think so.

Mike T.

 

This is an awesome thread, I am trying to achieve something similar. I have this circuit here that produces a short 1 second negative pulse through an Optocoupler. I'm not sure if it can be done but this circuit would be even more perfect if it could be made to reset after the ground from the DPDT rocker was applied???
Does anyone know a good way to do this?
Otherwise I would need a separate circuit like this for each one of my DPDT rockers, I have 5 rocker switches I am working with.