constant to momentary output info

Also should have asked how long the pulse input need to be to charge capacitor. My door lock unit only  give a 0.5s pulse

Sounds like you want a pulse stretcher, rather than a constant to momentary circuit. This setup won't work for what you're looking for.

It needs an electronic circuit to do it; just using relays won't work.
There are prebuilt units to output a timed pulse on a trigger input, or you could put together a DIY circuit, if you're willing to work with some solid state bits.

The reason why the above circuit won't do what you're looking for is because your input pulse is only 1/2 second. This setup needs an input that is switched on for a longer time than your desired pulse.

If you really want to do it with relays, you would need to add a latching relay so that your 1/2 sec input will be converted to a continuous "on" signal. Then you could use this setup, and increase the size of the cap to get a longer ON time. Use the 12V signal output from the timed relay to keep the first relay latched, so it'll reset and ready itself for the next pulse.
I generally wouldn't use this kind of setup for timed pulses (solid state driver will be much easier to get a long pulse than with huge capacitors), but if you prefer relays, it could be done that way.

Here are two options, take your pick-

relays only:


transistor driver:


If you have space for a HUGE cap, go for door number 1. You'll need to adjust your cap size depending on what relay you're using, if it draws more power, a bigger cap is needed for the same delay, and vice versa.

Door number two would be tiny and cost for the parts would be piddling.

Door number three is to buy the commercially available pulse timer, just more money but simplest.

Yes, it's a transistor- The BS170 is a MOSFET, which you can basically consider a voltage controlled switch. They're available online at most electronics places, you can look at www.mouser.com and just enter the part number in the search. Should be under 10 cents each; buy a hundred for a buck and you'll have enough for a lifetime :)

Just be careful when handling it as it will be more sensitive to static electricity until you get it all connected up in the circuit.

According to Mouser 100 of them will cost a whopping $12.  A buck will only get you a handful at $0.16 a piece!

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Kevin Pierson

Oops, don't know what I was thinking- I was at qty of 10, not 100.

Yeah, it'll cost more than a buck for that many. I got 100 for 10 cents a piece, looks like they went up to 12 cents now. I stocked up, so I don't look at the prices any more. I'm set for life anyway.

They're very handy as a general purpose switch; I accidentally blew out a relay driver chip on an alarm module, and I just wired in a transistor, using the signal from input to the blown driver to control it. Worked just as good as new.

Another use I had was to use it to control a wireless doorbell transmitter, driven from the normal household doorbell. I wanted to have the portable wireless doorbell controlled by the one button, so the transistor isolated the AC voltage from the transmitter.

It doesn't draw any current on the input, so it can be switched without any loading on the input signal. It's just a handy thing to have available for all sorts of applications, so I figured it would be worth the whole ten bucks.

So i am going to need 2 of these circuits to open and close the mirrors

Actually, the second circuit is set up for for a constant to pulse output, I'll make a few changes to tailor it for what you'll need. I'll draw it up and post it up in a bit-

When everything is sitting there before the trigger comes in, the capacitor is discharged.

When the voltage between the gate (pin 2) and the source (pin 3) of the transistor is low, it's off, and basically an open circuit between the drain (pin 1) and pin 3. The relay is not turned on as a result.

When your trigger comes in, the capacitor charges up to the input voltage through the diode D3, and the transistor turns on.
When on, the transistor connects the relay coil to ground, turning on the relay for the start of the output pulse.

With no more input from your trigger, the cap discharges through the R1 resistor, until the voltage drops below the turn on point of the transistor, and the relay opens up again.

You can use a larger capacitor or resistor to get a longer pulse time, and vice versa.

It should be in the range of 5 seconds; but just adjust the resistance to make it longer or shorter as required. You don't have to change both at the same time, just go up or down on either to change the timing.

You don't have to use the same values shown, if you happen to have some spare parts handy, just pick combinations that will give you roughly the time you want when you multiply R x C.
e.g., If you use a 500k resistor, it would drop the pulse time in half; if you use a 2M resistor, and it'd double the time.

Keep in mind the time of the pulse starts after your input pulse turns back off, so you need to add your input pulse time to the timing. If your input pulse was 2 seconds long, it just extends the pulse by the designated time.   If you put a constant (+) input to it, the output would go on and stay on until you remove the input.

The diode across the relay coil is needed to suppress the voltage spike that occurs when the relay is turned off; it would go well over the maximum voltage that the transistor can handle and destroy it otherwise.

The diode on the input allows the input pulse to pass in only one direction, in case your signal goes to ground on standby. If you didn't have the diode there, the charge on the cap would drain off back through the input instead of the resistor, affecting the timing of the pulse.