I found this voltage controlled switch circuit http://www.simplecircuitdiagram.com/201 … og-switch/. I’ve breadboarded it, but am not getting the OFF/ON capability - I’m probably misunderstanding the hookup for Vin and -12.
What I want to do is provide a 6V Vin that produces a 6V Vout, then when the Vin goes to 5V the Vout is switched OFF. This isn’t what I’m seeing with the circuit that I’ve attached. With switch open the LED is ON and Vout is 6V. If I close the switch and adjust the pot, Vout changes and the LED goes OFF or ON (no dimming). This seems correct. If at 6V Vin I adjust the pot so that the LED is ON, I’d expect the LED to go OFF when I change Vin to 5V, but I doesn’t - it dims. Plus the pot temperature soars from 27C to 40C (and climbing) before I open switch again.
Am I hooking this up wrong?
Is there a better way to have a voltage switch (maybe an IC)? If so, please recommend.
BTW: I had to substitute JFET PMBFJ108 for MPF102 because the 102 is no longer available.
Thanks for the comparator idea. But I want to stick with the JFET solution because it’s simpler and cheaper.
I learned that the original circuit I was basing my circuit on was incorrect. I found another page that discusses using a JFET as a voltage controlled switch, and modeled my circuit using it: http://www.allaboutcircuits.com/vol_3/chpt_5/2.html
The circuit is working fine. With V2 applied, the LED is ON. When I apply V1 it turns OFF.
lyndon:
I’m happy that it’s working for you, but what you ended up with was not what you said you wanted!
Yeah, I did a poor job in my original description. The last description is more accurate - a switch that turns a circuit OFF once a trigger voltage (V1) reaches a specific level is probably the best description.