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### Need help with a circuit again

Posted: Sat Sep 29, 2018 1:25 pm
This is a circuit the you guys have had helped out in the past, Its a Schmidt trigger , C1 determines the time the circuit will stay on until other led turns on, Right now the circuit has a 2 second delay. I was wondering if there was a way that I could incorporate a trim pot and a bleeder resistor with a bigger capacitor to adjust the amount of delay from 2 seconds to 15 seconds or anything else in between.

The only problem is I would like to add a picture of my schematic but I cant add any jpeg. bip, tif, pig attachments

any suggestions

### Re: Need help with a circuit again

Posted: Sat Sep 29, 2018 3:10 pm
I'll have to assume a resistor and C1 determines the 2 second time. As 2 seconds will be the minimum time, leave that resistor in the circuit. Add a potentiometer (configured as a variable resistor) in series. The total of existing resistor and variable at max should be (about) 7.5 times the value of existing resister alone.

555 time IC circuits also do this job well. (CMOS 555s can use lower power resistors).

Cheers,

### Re: Need help with a circuit again

Posted: Mon Oct 01, 2018 5:52 am

### Re: Need help with a circuit again

Posted: Mon Oct 01, 2018 8:19 am
I've seen this here before.

Don't make the capacitor bigger. Put a 74K (or nearest standard value potentiometer configured as a variable resistor in series with the 10K resistor. At zero the 10K gives the 2 seconds as before; at 74K setting the 10K plus 74K gives around 15 Seconds. In-between settings give in-between times.

Cheers,

### Re: Need help with a circuit again

Posted: Mon Oct 01, 2018 9:17 am
OK, The reason I thought that I needed to add a bigger capacitor is because when I changed it from a 22uf to a 10 uf capacitor the led stayed on for a 1/2 second .

### Re: Need help with a circuit again

Posted: Mon Oct 01, 2018 12:57 pm
See-
https://en.wikipedia.org/wiki/RC_time_constant

Going strictly by RC you can increase the time by increasing the capacitor. But, you've got zero ohms in the charge part of the circuit. That can be hard on the switch contacts, and maybe the power supply too. So I suggested increasing the resistance in the discharge part of the circuit to increase the "on" time.

Also, you said "adjust" from 2 to 15 Seconds. You will not find a variable capacitor anywhere near 22uF. ((Variable capacitors are like 500pF (aka 500uuF) a 100000 times smaller, and down.))

Cheers,

### Re: Need help with a circuit again

Posted: Mon Oct 01, 2018 5:26 pm
Thanks again, I think you helped me on the this circuit before,

### Re: Need help with a circuit again

Posted: Mon Oct 08, 2018 7:50 am
I breadboard that circuit last weekend, and it didn't work, depending on the resistance that I would set on the pot, the second light would stay on for maybe a couple of seconds, to much resistance and both lights would turn on real dim, then once I would turn of the switch the the second light would finally turn on. Also I should note that I took out the resistors out for the LED's and I am using incandescent lights.

### Re: Need help with a circuit again

Posted: Mon Oct 08, 2018 9:23 am
???
Where did you put it, and how much?

### Re: Need help with a circuit again

Posted: Mon Oct 08, 2018 9:30 am
I had a 1 m pot that I pot in front of the 10k resistor, then I tried a 10k , 47k resistors, any thing larger than that the circuit wouldn't work or it would act funny.

### Re: Need help with a circuit again

Posted: Mon Oct 08, 2018 10:25 am
I said 74K. 1M is 13 times bigger! Did you short the wiper to one end to make a variable resistor in series with 10K?

Or ground one end of pot (making a high resistance voltage divider (wrong!).

### Re: Need help with a circuit again

Posted: Mon Oct 08, 2018 1:22 pm
I will try it again I will letyou know what I get

### Re: Need help with a circuit again

Posted: Tue Oct 09, 2018 7:10 pm
Sorry I had it laid out wrong, the circuit works fine, I need to get a smaller pot, hoping to have the second light stay on a little longer after I disengage the switch, thanks

### Re: Need help with a circuit again

Posted: Mon Oct 15, 2018 4:58 am
Testing "Attachments". Might still be useful. The only change from previous circuit is pot in base's path of Q1.

Important - use "Inline" button to put attachment tags in the reply.

Cheers,

P.S. Thanks T&L/admin, should make some real happy. Works and was easy to figure out how to use.

### Re: Need help with a circuit again

Posted: Mon Oct 15, 2018 7:11 pm
Thanks again