Hello All,
I am new here. We just got a 09 "Just a Cooper" and love it.
I was looking for some electronics experts to chime in. For what I am doing the only signal I have to energize a relay has a slow rise from .0.0v to 12v. What I am looking for is a way to block the slow rise in voltage until it hits 12v +-, and then let it pass to energize a 12v relay.

I know I could build a logic gate or a Schmitt trigger, but if I can I would rather keep it KISS.

Thanks for your help.

 

Can you esplain more about what you are trying to do. There may be another circuit that you can use that has instant on voltage.

 

It's actually on my Jeep.

I am using the courtesy light circuit as a control signal to a relay. The relay will power my rock LED lights. I am doing this so when I open one of the doors the ground is lit up. It will come in handy at night trying to get out of the jeep. I got it all working so the open door will trip the relay or a switch when the doors are closed.

The issue I have is the Jeeps slow dimming of the courtesy lights when the key is out of the ignition (this is normal). The Jeeps signal to turn off the courtesy lights is a rise from 0.0v to 12v. The whole circuit is controlled by the PCM. This slow rise in voltage causes my rock light relay to shudder from about 3v to 9v where it latches. With the key in the ignition the fade does not happen so all is good that way.

I tried to fix this with a 1N4742A, 1-Watt 12v Zener Diode. My thought was the 12v reverse breakdown voltage would stop the rise in voltage from getting to the relay until it hit 12v; then break down and let the current pass to the relay. This did not work. I think the current might not be adequate to push the diode into break down; or the voltage loss across the diode will not let it get to 12v.

 

The Zener diode didn't work because as soon as the diode goes into breakdown, it does allow current to flow, but it also drops the voltage by 12V, leaving nothing to activate the relay.

I can't think of a way to do what you need using only passive components (resistors, capacitors, inductors, etcetera). It would be trivial to do it with a single op-amp set up as a comparator, so that when the input voltage gets over 10V, the output voltage instantly snaps from 0V to 12V, but the problem is that an op-amp likely won't source enough current to activate your relay. A standard Bosch-type automotive relay needs about 160 mA to activate, and a typical op-amp like the LM741 can only source about 25 mA.

If you're willing to use an op-amp and a transistor, you can use the op-amp to trigger the transistor, and use the transistor output to activate the relay.

I'll keep thinking on it and see if I can come up with some way to do it with passive components.

 

I was thinking of trying a resistor and cap in series, and let the voltage rise charge the cap. Once the cap is charged it can dump to the relay.

 

Unfortunately, you'd need a way to isolate the capacitor from the relay input until the capacitor has charged to at least +9V. Otherwise, as soon as the capacitor reaches +3V, your relay will start to oscillate just like it's doing now.

I still can't think of a simple passive circuit that will take a slow-rising transition and convert it to a sharp-edged transition, which is what you need to keep the relay from oscillating. I'll keep thinking on it, though.

 

Thanks for your help. If you have time; would you mind sketching out the op-amp and transistor idea?

 

Sure - it'll take me a day or so since I have to research single-supply op amps. Most times, you'd power an op amp with equal-and-opposite power supplies, like +/- 12V or +/- 5V. But since you only have access to +12V and ground, not -12V, that changes the circuit a little bit. Still fairly straightforward, though. Is your relay the standard Bosch-type SPDT or SPST relay that you get with most light kits?

Scott

 

Thanks, yes it's a stanard NO SPST 12v 30amp relay.

 

Okay, here's what I came up with:



The divider network R1/R2 gives +6V on pin 3 of the LM311 to act as a reference voltage. V0 is the signal from your Jeep that goes from 0V to 12V.

When V0 is below +6V, the output from the 311 will be low (about 0.2V). When V0 goes over 6V, the output from the 311 will go high (about 12V), turning on the transistor.

The value of RE (the transistor emitter resistor) determines the emitter current, which will be the same as the collector current through your relay coil. If the base voltage on the transistor is about 12V, the emitter voltage will be about 0.6V less than that, or 11.4V. 11.4V through RE will give a current of about 190 mA, which means that the current through the coil relay will be about 190 mA, which should be enough to turn it on.

R3 is just a pull-up resistor - I chose 10k to keep the current from the 311 low.

IMPORTANT - R1/R2/R3 can be regular 1/4-Watt or 1/2-Watt resistors, but RE *must be* a power resistor. Since the emitter current is about 190 mA and RE is 60 Ω, RE is going to dissipate about 2W while your rock lights are on. I'd get a couple of 10 Ω and 20 Ω power resistors in addition to something close to 60 Ω so that you can play around with the value of RE in case you need to adjust the coil current.

You should have no problems finding an LM311 for the voltage comparator. For the transistor, I built the circuit and tested it using a 2N3904, but 200 mA is about the maximum for that transistor, so that's why I'm recommending a 2N2222 instead, since they can handle 200 mA (or more) easily.

The SPST device on the schematic is your light relay. Pins 85 & 86 should be the coil connections on your relay.

12V on the diagram is +12V from your Jeep, and GND is a chassis ground on your Jeep. Of course you can tie all three ground connections together to a single chassis ground.

This circuit will draw a little bit of power constantly, but it's on the order of a few milliwatts, less than an alarm system or even the clock/computer in your Jeep.

Let me know if this all makes sense and if you have any questions.

Scott

 

That looks great. I will give it a try. Dose not look to hard to build.

 

Why dont you just bypass the capacitor circuit in your dome lights?

Or you can use a transistor as a switch. You have to Bias the transistor so that it turns on at a low input voltage. Then you have no need for an op-amp etc.

 

When he posted his problem, he said that the delayed turn-off of the courtesy lights was computer-controlled. I don't know the details of how the circuit works in a Jeep, so I couldn't help him with finding/bypassing the delay circuit.

As for the comparator I used to provide the base voltage for the transistor, I went that route because it doesn't take much base-emitter voltage (6/10th of a volt) to turn the transistor on, and I was afraid he might have a hard time "shooting the gap" with setting the transistor bias so that it would normally be off, but still turn the relay on quickly and reliably when the signal voltage started to rise.

I also don't know how long it takes for the signal voltage to go from 0V to 12V, and I didn't want him to have the same relay oscillation problem he's having now. With the voltage comparator, the transistor base voltage will rise virtually instantly from about 0.2V to 12V when the input signal voltage passes 6V, eliminating the potential for relay "chattering".

Lastly, I've modified the circuit since I posted it here to eliminate the need for the transistor emitter resistor, so the circuit is now comprised of just the comparator, transistor, and three common 1/2-Watt or 1/4-Watt resistors.

 

Thanks for you comment. I have put a VM on every point of the circuit I could find. Even the door pins are not a 12v on-off. I think it’s controlled by the PCM. I have to wait until I go into the city to get the LM311N chip that ScottRiqui has in the circuit.

 

If you'd like to try something in the meantime, here's an alternate solution that might work and doesn't require the LM311:

Connect the signal from the car to the base of the transistor with a 4.7 kΩ resistor (1/2 or 1/4 Watt - nothing fancy). Ground the emitter of the transistor and connect up the coil contacts to the collector of the transistor as shown in the diagram above.

This should turn on the relay when the signal voltage from the Jeep goes over 0.6V. The only potential problem is that the relay coil current won't immediately go to its full value - it will still "ramp up" a little bit as the signal from the Jeep approaches 12V.

I don't know whether the current increase will be fast enough to prevent chattering in the relay, but it would be quick and easy to test out.