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12V trigger

epsilon24

12V trigger

Postby mpa2 » December 11th, 2017, 2:03 pm

Hi,

I was looking for a possibility to switch ε24 (actually, β24) on and off using 12V trigger signal, since ε24 doesn't offer this feature. Ti's idea was to drive a relay which would act as a momentary switch for the ε24.

Played with NE555 first, but couldn't produce an impulse when switching off.

Here is a possible workaround (found this circuit and adjusted it a bit):

12V-Trigger.png


The relay engages for a second when 12V voltage is applied or removed and can trigger ε24. It seems to work fine, unless the voltage fades out, then there is no action.

Any suggestions for improvement?
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Re: 12V trigger

Postby amb » December 12th, 2017, 12:37 am

Your circuit is simple, but has the disadvantage of not working when the input has a long fall time. Below is an alternate circuit using a Single 2-input XOR gate with Schmidt Trigger inputs:

dual_edge_trigger.png

This circuit is even simpler in terms of parts count. An ideal XOR gate to use is the SN74AHC1G86. The Schmidt Trigger makes it tolerate slow rising or falling inputs, and helps to prevent false triggers.

You'll need to adjust the value of R1 and/or C1 to make the output pulses wide enough to trigger the relay reliably. A voltage divider at the input is needed to drop the 12V trigger voltage down to 5V (or 3.3V). Also, note that this XOR gate's output can source or sink only 8mA of current (at 5V), so you should add a transistor to drive the relay. Lastly, you should add a reverse diode across the relay coil to prevent back-EMF from damaging the circuit.
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Re: 12V trigger

Postby mpa2 » December 12th, 2017, 3:45 am

It sounds interesting, but would need an additional power supply, wouldn't it? One could use 12V from the ε24, the first circuit is powered from the trigger voltage alone though.

If we start to use logic, we could implement a check for the current power state of the ε24 and thus obtain reliable power on and power off signals. We need more gates then :)
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Re: 12V trigger

Postby amb » December 12th, 2017, 4:35 am

mpa2 wrote:It sounds interesting, but would need an additional power supply, wouldn't it?

A small SOT-23 voltage regulator to drop from ε24's 12V to 5V would take care of that. I don't know how much current a typical 12V trigger source could provide, but I wouldn't use it to power the relay.
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Re: 12V trigger

Postby mpa2 » December 12th, 2017, 12:53 pm

amb wrote:
mpa2 wrote:It sounds interesting, but would need an additional power supply, wouldn't it?

A small SOT-23 voltage regulator to drop from ε24's 12V to 5V would take care of that. I don't know how much current a typical 12V trigger source could provide, but I wouldn't use it to power the relay.


My source provides up to 80 mA, it worked with the first circuit.

Just for fun, could this work? It's probably to complicated and maybe it's just reinvention of the wheel, but who knows :) It should switch the amp on when it's of and vice versa.

Trigger with logic.png
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Re: 12V trigger

Postby amb » December 12th, 2017, 1:47 pm

Yes, this appears to solve a problem where the 12V trigger might do the reverse of what's intended (i.e., trigger on would turn off the amp). I haven't looked carefully enough at the schematic to verify the logic is correct, but it's easy enough to breadboard it and test.
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Re: 12V trigger

Postby mpa2 » December 14th, 2017, 11:23 am

Yeah, breadbord and SMD :) Do you know any through hole XOR 7486's with schmitt trigger inputs? I couldn't find any so far...
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Re: 12V trigger

Postby amb » December 14th, 2017, 2:56 pm

You can use SOT23 to DIP adapters like this:
https://www.sparkfun.com/products/717

There are adapters for other SMD packages too.
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Re: 12V trigger

Postby mpa2 » December 21st, 2017, 3:19 pm

So, this one seems to work on a breadboard:

12V Trigger_1_lowQ.png


My experience in circuit and layout design is zero, and the last time I used eagle was about 15 years ago, so be patient :)

Is it allowed to connect outputs of gates IC1A and IC1B this way or should I rather use an additional OR gate and a base resistor?

PCB could look like this, the same footprint as ε24:

board_low.png
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Re: 12V trigger

Postby amb » December 22nd, 2017, 12:32 am

Since this is low speed stuff, PCB layout is not critical. But you should put a 0.1uF MLCC next to IC1 and IC2 as supply rail decoupling.
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