Trigger for Amplifier

[Lovan]

Hello everyone!

I'm thinking of making a trigger signal to power on my amplifiers like you have on car amps.
I wan't the speakers to be turned on a little while(a few seconds) after the powerrelays have been shorted. Any idea how?

This is what I got so far.
R6 and C1 is to get some time for the speaker relay to turn off before the powerrelays turn off.



Thank you

/Lovan

[Lovan]

Ideas someone?

[Mooly]

Hi,
It's not very clear what you are trying to do Are you just switching low voltage rails ( that's what it looks like in the circuit ).
The time constant C1/R6 needs to be on the input to the gate not the output. It's to ill defined as it is, relying on "large" value electrolytics and the sink/source characteristics of the gate itself. Also the base volts to Q1 will not switch cleanly.
It's good practice to ensure any time constants "reset" instantly so that a very quick mains on/off sequence ensures it goes through the full switch on delay routine. Add a high value ( 1 to 10 meg) across the gate to speed up switching transistions, or use a shmidtt trigger type gate. Perhaps also a bit of filtering (debouncing) on pin 1.

[Lovan]

Hi!

Sorry for being unclear.
Yes the powerlines are +24V/gnd/gnd/-24V.
If I put C1/R6 on the AND-gate then it will delay both the upper and the lower AND gate. I wanted to isolate the signal to the lower AND-gate so to have a delay before the powerrelay was turned off. But maybe by putting a diode in series to isolate the lower AND-gate...

Schmitt triggers is a good idea. I'll look into that.

[Mooly]

Like this Use high value R and low C now. The diode show how to reset the delay instantly as soon as the trigger goes to zero -- if you want it to, but it sounds like you don't so leave out of course.

Edit, One other thing -- pin 1 of the IC. If thats 24 volts on there it's way to high. You need to feed it through a resistor ( say 220K ) and either have a 4.7 volt zener to ground or a IN4148 clamping it to the plus 5 volts. And don't leave any unused inputs floating !

[Lovan]

Hi!

Sorry for the late reply.

Yes I didn't put in any zener to lower from 24V to 5V but I was aware of the problem. I probably will put a zener clamping it to 5V and have a RC network to have a turn on delay.

I will put up a more up to date schematic tonight if I have time.

Thank you very much for your help

[Lovan]

Hi again.

Would this work? Those are NAND gates with Schmitt triggers. Quads in one package

Recommended values for C1/R2 and C3/R3?
Maybe put a diode over R2 to make startup faster?

[Mooly]

Hi,
Would it work ? I think so A few thoughts,
See you have it down as 74xx TTL logic. I would use CMOS, better defined logic levels, better noise immunity, allows you to use small non polarised caps for timing etc.
Trigger input. The power relay has to close quickly on switch on and have a delay on switch off. The diode across R2 allows the cap to charge "instantly" and, on switching off gives a delay defined by C1 and R2. Easy
Speaker relay, R3 and C3 and zener D2 give the delay on switch on. Do you need D3 ? Depends how the rails collapse, if you keep it I would add a 1 k or similar as shown. On the face of it this does nothing, but as rails switch strange things can happen. The junction capacitance of D3 could "spike" the IC and zener. All a bit unlikely I know but D2 and D3 are two potentially low impedance paths to ground across a high current rail.
Timing values, with CMOS you can use up to 10 Meg ohm with small caps such as 0.1 mfd polyester etc.
The trigger input ! When the amp is off this must not "float" so maybe add a pull down resistor to be sure.

[Lovan]

Hi!

I redrew the schematic and made a double layered board.







Wouldn't it be better to have it connected like this so that the positive voltage isn't affecting the delay time?



According to the datasheet the gate will go positive at about 1.3V. I tried to calculate the value of RC to get a 3s delay but got values of 1 µF and 1.5 MOhm

[Mooly]

Hi,
Bit of a rethink. Your last point on the rail voltage affecting the delay. That as drawn at the bottom will not work, the zener is directly across the rail (via D2 ). As it is in the circuit, that would work OK, but does as you say depend a bit on how the rail collapes.
What about mirroring the power relay part but with different time constants. Reversing the diode will give a delay on connecting the speaker but give an instant disconnect.
Also the drive to the transistors is relying on the short circuit ability of the IC -- add base resistors.

Edit -- The DIY board not accepting pictures today, try again later.