Do I *really* need a zener diode?

Hi, I want to construct the circuit below, described at this link: .

But I don't have a zener diode in my parts box, and frankly reading the wikipedia article on zener diodes, I can't quite make out what the point of it is in this circuit. Is it possible/safe to construct this without the zener, perhaps using a regular diode?

Zener diodes work like normal diodes until they reach their breakout voltage, which happens to be 5.1 volts on the model this circuit calls for. The Arduino also happens to run at 5 volts. So, given the other end is connected to ground, and no power is coming from ground, I doubt it's doing much more than a little voltage safety. You could probably use a normal diode, but it wouldn't be as safe. I'm not sure what kind of voltages are possible on the LANC wires, but if someone put the zener diode in there, I would use one.

Yes it's required. That circuit is a little more complex then may first appear. Note that the signal at the bottom of R2 is both a arduino input signal and a arduino controlled output signal. Also not that the output signal when turned off (TI cutoff) will rise to whatever voltage is on 'ring' pin specified as 5 to 8vdc. As anything over 6vdc will damage the arduino input pin the zener is needed as an active component to make the circuit function is a non-destructive manner.


Ok, perfect, thanks. I built it without, and it's working great right now. Tomorrow I'll run out and get the diode, and if I blow up my mcu tonight, it's the price to pay for impatience :grin:

Its actually probably rather inadequate as protection - the Arduino pins must not go above Vdd+0.5V, so you really need a schottky diode to Vdd (and a series resistor in the line to limit current). 5.1 Zeners are ~5.1V at their "nominal current", but can be a lot higher with high current pulses (low voltage zeners have soft knees). A schottky to Vdd will prevent the input rising above Vdd+0.5V much more reliably (even when the Arduino is powered down).

How should that be wired, exactly? Also, the signal voltage coming from outside seems to be 5-8v. What value of resistor should be used in this case? I do have schottky diodes, so that'd be handy too.


With the 4.7KOhm resistor the 5.1 zener will work just fine. The resistor will limit the current and protect the Arduino. WOrst case - the 4.7K resistor wwill allow something less than 2mA to flow.