On/Off switch by a voltage threshold - Possible SSR use

[fizzy123]

I thought you said you wanted it to switch on at +12V and ignore the +5V pulse, ie. stay switched on during the period when the voltage dropped to +5V .

If so, an SSR with a +4V input threshold, will switch off below +4V (0V is below +4V), and switch on above 4V (+5V or +12V).

Does that make sense?

No it's the opposite that I said. Let's say that turn signal is off. In that off state controller keeps sending 10ms 5v pulses every 30 secs. That results in brief LED flash. I do not want that. I want the turn signal LEDs to stay off at +5V. The LEDs must come on only when the 12V is applied. So my desired setup is:

0V = Off
5V= Off
12V= On

[pebe]

Sorry, I misunderstood you. I thought the 5V pulses were occurring during the 12V on period. Your mention of a delay being a possible solution threw me.

Back to my original scheme. Best to use the 741 circuit I suggested before, but I'll modify it so the relay switches on when the indicator is switched on. Circuit will follow ASAP.

[fizzy123]

Sorry, I misunderstood you. I thought the 5V pulses were occurring during the 12V on period. Your mention of a delay being a possible solution threw me.

Back to my original scheme. Best to use the 741 circuit I suggested before, but I'll modify it so the relay switches on when the indicator is switched on. Circuit will follow ASAP.

Great thanks for help. Look forward to the circuit. As soon as it triggers SSR above 8-9v, it will does the job.

[pebe]

OK here's the circuit. Relay is 12VDC type and contact only needs to be single pole type. 'NO' is the Normally Open contact on the relay. You can use just about any type of op-amp if a 741 is not available - but do NOT use a comparator.

I think it is self explanatory, but shout if it isn't.

[fizzy123]

Ok I think it is a bit difficult for me to work out what's happening there. specially I am struggling with the PIN numbers on op amp. If you take my cartoon circuit and add the additional resistors and op amp there without internal circuitry that will be easier for me

Between I see you are in Ellon near Aberdeen. I am in Kingswells.

[colin55]

You don't need an op-amp, just a 5v6 zener on the input line.

[fizzy123]

You don't need an op-amp, just a 5v6 zener on the input line.

Yes that was the conclusion but last 3-4 posts on this thread are missing since the server maintenance. Thanks for an accurate and to the point reply

[fizzy123]

Ok an update. Looks like the 10ms signal I was trying to suppress is not 5v but also 12v. This means that Zener diode or resistor will not resolve this issue. Sorry for mis information but I receivd it from some where else and the pulse is so brief that it was not possible to confirm it with a meter.

The next thing I can do is to delay SSR by just over 10ms. Any suggestions how to do that?

[pebe]

Yes. Easily done with an RC combination. I'll work out the values.

(EDIT: I think we have lost the page where you said what the input resistance was. I shall need to know that.)

[fizzy123]

Yes. Easily done with an RC combination. I'll work out the values.

Great man. Genius

[pebe]

See edit

[fizzy123]

See edit

I think the resistance at the input is around 560ohms.

[pebe]

OK. Put a resistor of 560ohms in series with the input to the SSR, and a 100µF or 220µF cap from SSR input to battery negative.

A 12V 10ms pulse will not cause the SSR to turn on because the cap voltage will to rise to only 1.5V (even less with a 220µF cap),

The 560ohm resistor will reduce the pulse to 6V at the SSR input, but it will still switch normally.

[fizzy123]

OK. Put a resistor of 560ohms in series with the input to the SSR, and a 100µF or 220µF cap from SSR input to battery negative.

A 12V 10ms pulse will not cause the SSR to turn on because the cap voltage will to rise to only 1.5V (even less with a 220µF cap),

The 560ohm resistor will reduce the pulse to 6V at the SSR input, but it will still switch normally.

That sounds simple enough. Is a capacitor of specific voltage / current rating to be chosen or any cheap 100µF or 220µF capacitor off ebay will do the job?

And for my knowledge only, how a capacitor helps to skip a 10ms pulse? (Probably it is the time capacitor takes to store the current before discharge? So I assume bigger the cap value, longer the delay) How the calculations work out at 12v?

[pebe]

First, let me correct my error. The size of the cap should have been 220µF, or preferably 470µF.

The reasoning is this. At the beginning of the pulse, the cap is discharged. When the pulse arrives the cap will start to charge via the 560ohm resistor. The voltage across it will rise exponentially with time and the final charge would be 6V. But it will only be charging for 10ms and by that time we want the voltage to have increased to no more than 1.5V, so the SSR won't fire. To get to that voltage in 10mS means the time constant (T) needs to be 50ms.

The time constant (T) is given by C x R, where C is in microfarads and R is in megohms. and T is in seconds. So CR= T =50ms. Transposing for C gives C = 50ms/R. I had taken R as 560 but R is actually the input impedance of the voltage source, and that is the 560ohms resistor and the 560ohms input resistance in parallel., which is 280ohms.

So the capacity C will work out at 0.05sec/0.000280megohms = 178.5µF. The minimum practical value is 220µF, but 470µF would be better. C could be any elecrolytic, but preferably could be a low ESR type

I hope that answers your question.