Limiting voltage using Zener diodes.

[Six_Shooter]

Here's the situation:

I have a 2000 GMC Yukon that has factory CD with remote cassette.

Well there is no auxiliary input and want to feed the output from my iPhone or iPod, or hopefully as soon as I get a Samasung phone...

Anyway, what I'd like to do is replace the slave cassette unit with an aux input. The audio signals are easy enough, they are easily connected.

What people have been doing is just leaving the cassette in place and when they want to listen to their aux devices, they just push a cassette into the player to trigger the main uit to switch over, hokey.

On the back of the unit is several connections, one of which is called an E&C pin (Entertainment and Convenience), that sends a certain pattern of pulses to the main unit when the cassette is engaged and then a different pattern when it is ejected.

Here's the issue:

I have a PC based Velleman scope that I can't seem to get to trigger as a single shot, for whatever reason. I do however have a Digiview logic probe. Sounds like a simple fix, right? Just connect the logic probe and capture the output, but not so fast. The output of the E&C pin I have measured at about 14V, the logic probe is only good up to 5V and I have been warned to head that limit very carefully with this particular unit.

So what I have come up with is a simple zener diode circuit, that according to Multisim seems to do exactly what I need it to, at least with a static input voltage of 14V.

The circuit is simply a 4.7K ohm resistor in series with two 1N5222B zener diodes. With these values I should have just under 2mA for IZT, which is much less than the max of 500mA that these diodes are rated for.

Is this a good solution? Will this allow for waveform capture, that happens very quickly without allow any over voltage to the logic probe?

TLDR; Trying to limit voltage input to my logic probe, wondering is a series resistor zener diode circuit is a viable solution?

 

[Harald Kapp]

This setup should work.
A resistive divider (2k+1k) could do the job, too and preserves the waveform (which is irrelevant fpr the logic probe but might come in handy if you use a scope).

 

[Six_Shooter]

True, I hadn't thought about the effect of the diode on the waveform.

I just sdid some more testing in Multisim, I forgot about the Function generator in the program *facepalm*.

So I set the FG to 14V Vp and 32 kHz square wave (typical IR carrier frequency, though I don't know what the frequency of the output of the target device is yet, this seemed like a starting point.

Anyway, the voltage then jumped to 6.39V, I took one of the diodes out and the voltage dropped in half, as expected.

I figured the zener would have been a better choice over the voltage divider, due to the voltage divider just simply being a ratio of Vi to Vo, and the zener should actually clamp it to a certain voltage regardless of the input voltage (within reason).

Maybe I'll build it up on a breadboard and see what I can measure. It seems I need to select a different diode though, to stay below that 5V point, but be above 4V to get a good trigger.

 

[Harald Kapp]

You can combine both approaches:
Use the divider to obtain a voltage between 0-5V during normal operation. Addd the zener diodes in parallel to the lower resistor of the divider to obtain protection from spurious overvoltages outside the normal operating range. Thus you maintain the waveform, clipping only overvoltages.