Timer Circuit Help

[behrendfry]

I am designing a simple circuit used to test the operating life of our TWS sights. The concept is we attach the LDR to the eyecup and turn the system on, once this is down we use the push button to activate the clock. When the battery dies on the system the LCD screen of course goes black and changes the resistance in the LDR and it activates the relay which acts just like the push button to stop the clock. As it is the circuit works great, but I found an issue, after a brief moment of the system being powered down the battery gains enough strength to power it up again for 30-40 seconds which does nothing to the clock but when it shuts down again it triggers the relay which will start the clock back up, this is no good. Is there a way or a device that once the relay is triggered once it cannot be triggered again without doing it manually? I was thinking maybe a SCR (silicon control rectifier). Any suggestions would be helpful. Schematic is attached for your viewing.

 

[Roff]

You can try an SCR in place of your transistor, but the most sensitive one I could find (in a short search) was **broken link removed** from Teccor, and its minimum trigger current is 1ma. It might not work reliably in your circuit. You could try to find a more sensitive one, or you could add positive feedback to your existing circuit, as below.

You could also possibly use a latching relay, but you would have to add a way to unlatch it.

 

[ljcox]

Your circuit is rather crude. I would have used a Schmitt Trigger to provide a LDR threshold.

As for your question, the relay can be latched by connecting a contact (an NO one) across the transistor. Thus once it operates it will not release until the power is removed. Alternatively, the second contact (an NC one) could be in series with the LDR so that, once the relay operates, the LRD is disconnected.

 

[Roff]

Your circuit is rather crude. I would have used a Schmitt Trigger to provide a LDR threshold.

As for your question, the relay can be latched by connecting a contact (an NO one) across the transistor. Thus once it operates it will not release until the power is removed. Alternatively, the second contact (an NC one) could be in series with the LDR so that, once the relay operates, the LRD is disconnected.

Good points, Len - although if you add latching feedback, you shouldn't need a Schmitt trigger. The latch has the ultimate hysteresis.

 

[ljcox]

Good points, Len - although if you add latching feedback, you shouldn't need a Schmitt trigger. The latch has the ultimate hysteresis.

I agree about the hystersis Ron, but the other reason whu I would use a Schmitt is because the "threshold" of the transistor will vary with temperature and device.

Have a good Xmas.

 

[Roff]

Good points, Len - although if you add latching feedback, you shouldn't need a Schmitt trigger. The latch has the ultimate hysteresis.

I agree about the hystersis Ron, but the other reason whu I would use a Schmitt is because the "threshold" of the transistor will vary with temperature and device.

Have a good Xmas.

You're just full of good points ain'tcha?
I suspect the LDR will also vary with temperature. We'd better be careful, or we'll just conclude that it can't possibly work.

Merry Christmas to you too - and all you other trolls out there. :lol: