Novice question, how to turn an SCR off

[eeechuta]

So i want to build a device that, when the light level outside gets above a certain level, it trips a Solenoid that closes my window shades.

As far as i can tell, the best way to do this is to use a photoresistor (in series with a potentiometer for adjustability) to trigger the gate on an SCR and allow (rectified) AC to flow through my solenoid. The problem i can't get past, however, is how to turn the SCR off, which i would like to do rather than run 120 volts all the time once the scr is tripped.

The solenoid isn't the important part, the main part is how do i MOMENTARILY allow AC current to flow and automatically turn itself off (to disengage the SCR)?

Am i on the right track? can i use a DC signal from my photocell circuit to trigger an SCR attatched to an AC source?

Any input will be most helpful.

 

[Gene]

Your trigger should work. As to the disengage, maybe a microswitch on the windowsill that would break the circuit when the shade lowers. The bottom of the shade presses down on the N.C. switch when the shade is all the way down and turns off the switching circuit. Another circuit (wired in reverse) could open the shade when the sun comes up. Sounds like fun.

 

[eeechuta]

That's a good idea, I didn't think of using a switch. It would be nice to keep it all electronic, though, if there is any way to break the circuit electronically. Otherwise, i'll probably go with the mechanical switch idea though. Thanks!

 

[stevez]

I've seen SCR circuits arranged so that a transistor momentarily shorts the SCR circuit so it looks like the power is off and the SCR stops conducting. That way you keep it electronic.

 

[eeechuta]

How would i go about getting the transistor to short the SCR? maybe i'm just not following you.

 

[stevez]

I recall seeing this done on some Honeywell building automation equipment where a short duration signal triggered the "on" state on a box attached to the equipment or point being controlled - and it stayed on without need for further communication. The technician explained, while showing me the diagram, how another pulse momentarily turned a transistor "on" - the transistor being wired to effectively short the load terminals of the SCR forcing it to an off state. The SCR powered another relay or motor starter. Sorry I don't have more details - maybe others can help here.

 

[Gene]

You could use your SCR circuit to drive a "one shot" timer centered around a 555 IC. The timer could control how long the shade lowering motor runs. Of course, the microswitch would be easier, cheaper, and more dependable.

 

[lavenatti]

The SCR is going to stop conducting every half cycle if you are running AC through it. You don't have to worry about shutting it off. All you need to do is apply a gate current long enough for your shades to close.

 

[Sebi]

SCR = thyristor. If You apply in AC circuit, the load (motor) is half-powered, because the SCR is one-way element. Use a TRIAC with optotriac trigger (for zero-cross switching, and isolation) or (for low-power load) with DC trigger.