Hi Aim65. Sorry for such a late reply. Yes the trimmer and the cap set the trigger time. Once the delay is initiated the relay is grounded through the output pin of the 555, engaging it.
I feel that the reason why the turn off isn't instantaneous is because to the cap holding charge. I'm wondering if a smaller RC combo might allow for the cap to discharge quick enough to close the relay for turn off or else maybe I can use a discharge resistor?
I've used this scheme before without trouble, but in the future the MOSFET looks like a good idea too.
I feel that the reason why the turn off isn't instantaneous is because to the cap holding charge. I'm wondering if a smaller RC combo might allow for the cap to discharge quick enough to close the relay for turn off or else maybe I can use a discharge resistor?
I've used this scheme before without trouble, but in the future the MOSFET looks like a good idea too.
Hi DylanBenton,
Yes, the charge held in the reservoir cap is the cause, but you can’t avoid this. To avoid the ‘plop’ in the speaker, you have to disconnect the speaker when the power start shutting down. In other word, you have to trigger the 555 (with a falling edge on TRIG pin) when the supply voltage (+36V) goes below a fixed level (let’s say +33V); when 555 is triggered, output goes high and relay goes off. If your circuit 555 woks ok, keep it and add a low supply detection.
You can try the following circuit, which is a part of the one in the previous post, and is connected to your existing 555 circuit.
The circuit around T1 & T2 detect low voltage on power supply (close to 33V) and trigger the 555.
This is not very ‘clean’ as T2 short a capacitor, but it should survive. If 33V is too high, and you encounter unexpected relay switch off, try decreasing R7 value to lower the threshold point.
Chris
Yes, the charge held in the reservoir cap is the cause, but you can’t avoid this. To avoid the ‘plop’ in the speaker, you have to disconnect the speaker when the power start shutting down. In other word, you have to trigger the 555 (with a falling edge on TRIG pin) when the supply voltage (+36V) goes below a fixed level (let’s say +33V); when 555 is triggered, output goes high and relay goes off. If your circuit 555 woks ok, keep it and add a low supply detection.
You can try the following circuit, which is a part of the one in the previous post, and is connected to your existing 555 circuit.
The circuit around T1 & T2 detect low voltage on power supply (close to 33V) and trigger the 555.
This is not very ‘clean’ as T2 short a capacitor, but it should survive. If 33V is too high, and you encounter unexpected relay switch off, try decreasing R7 value to lower the threshold point.
Chris