zener diodes

[adam-ads]

I'm getting confused about how zener diodes are rated and could use some help.

I have a Motorola 2-way handheld radio that I need to wire for an external speaker. The connector on the radio has a pin which is used to select between internal and external audio.

The pin is tied high to +5V by a 20k Ohm resistor and needs to be pulled down to 1.24V to enable external audio.

So, am I looking for a 1.24V zener or a 3.76? Does the amount of current flowing through the resistor come into play?

 

[ConradH]

Consult the docs, but the way things typically work, a pin will be pulled up by a high value resistor. You simply pull it down to ground to activate the function. The resistor limits the current to a low safe value. I assume they just mean that the pin has to be pulled below 1.24V to be sure the function works. That 1.24V is too low for zeners, though there are some references at 1.2V, but I don't think you need anything like that. If for some odd reason you did, the 20k is high enough that literally any part would work.

 

[Hyperion]

Consult the docs, but the way things typically work, a pin will be pulled up by a high value resistor. You simply pull it down to ground to activate the function. The resistor limits the current to a low safe value. I assume they just mean that the pin has to be pulled below 1.24V to be sure the function works. That 1.24V is too low for zeners, though there are some references at 1.2V, but I don't think you need anything like that. If for some odd reason you did, the 20k is high enough that literally any part would work.

A couple of ordinary diodes in series ?

 

[adam-ads]

Consult the docs, but the way things typically work, a pin will be pulled up by a high value resistor. You simply pull it down to ground to activate the function. The resistor limits the current to a low safe value. I assume they just mean that the pin has to be pulled below 1.24V to be sure the function works. That 1.24V is too low for zeners, though there are some references at 1.2V, but I don't think you need anything like that. If for some odd reason you did, the 20k is high enough that literally any part would work.

Thanks.


The info I have says that the control pin us used to select one of four possible modes:

All Internal, STD Operation 5VDC
EXT MIC and SPKR, STD ANT 1.24VDC
EXT MIC, SPKR and ANT 2.5VDC
EXT ANT ONLY 3.74VDC

Unfortunately, the documentation I have is limited.

The mode I want is the one with the lowest voltage (1.24V), so maybe I could just short it to ground. Does that make sense?

There's also a B+ pin on the connector but I don't yet know what voltage that is or whether I should be using it to control the mode selection.

 

[Jon_Logan]

A couple of ordinary diodes in series ?

+ 1 to that.

It sounds like the pin that the diode(s) are tied to, is just a decision circuit.
The circuit switches to external audio mode when the voltage at that in is below some voltage. Without the data sheet, I'd avoid fully grounding that pin. As Hyperion say's two diodes in series will do what you want. You point them in the opposite direction of a zener diode. Two, in series, Cathode (or stripe, band, or K) pointed at ground.

By the way, aimed at Hyperion. Proud day for the Brits. Opening Ceremonies were awesome!

 

[adam-ads]

OK, so two ordinary switching diodes such as 1N914, in series to ground, should yield about 1.2V at the control pin. Is that correct?

Thanks for the help, guys!

 

[Jon_Logan]

OK, so two ordinary switching diodes such as 1N914, in series to ground, should yield about 1.2V at the control pin. Is that correct?

Yes. That should put it into external audio mode.

 

[djoffe]

Granted, this is a guess, but here goes:

the receiver circuit on the pin has decision points at 1.5, 3.0, and 4.5 volts.

That allows it to determine 4 conditions
1. less than 1.5 (1.25 satisfies this)
2. more than 1.5 but less than 3.0 (2.5 satisfies this)
3. more than 3 but less than 4.5 (3.75 satisfies this)
4. more than 4.5 (5 satisfies this)

I'd be very surprised if the system objected at all to grounding the input pin to achieve condition 1, but then, you must do what makes you feel comfortable.

www.updatemydynaco.com

 

[adam-ads]

Thanks again. I'll give it a shot with two ordinary diodes.

 

[Hyperion]

+ 1 to that.

It sounds like the pin that the diode(s) are tied to, is just a decision circuit.
The circuit switches to external audio mode when the voltage at that in is below some voltage. Without the data sheet, I'd avoid fully grounding that pin. As Hyperion say's two diodes in series will do what you want. You point them in the opposite direction of a zener diode. Two, in series, Cathode (or stripe, band, or K) pointed at ground.

By the way, aimed at Hyperion. Proud day for the Brits. Opening Ceremonies were awesome!

Thanks Jon, I thought it was bloody excellent too. I don't want to de-rail the thread though.

Back to business.
2 diodes = good :thmbsp:

 

[adam-ads]

+1 for the opening ceremonies. I especially liked Mr. Bean and the old lady with the handbag

 

[ConradH]

Well, it looks like an "intelligent" pin! You can use the diodes or a 1.2V reference chip (overkill), or since the pin is pulled up by a known resistor you might even get away with just choosing a loading resistor that gives the desired voltage for the function you want.

Is there anything made by the manufacturer that plugs in and does the same selection? If so, get a schematic and see how they did it. If not, I very much doubt you can hurt it, so long as it's just a pulled up pin.

 

[adam-ads]

Well, it looks like an "intelligent" pin! You can use the diodes or a 1.2V reference chip (overkill), or since the pin is pulled up by a known resistor you might even get away with just choosing a loading resistor that gives the desired voltage for the function you want.

Is there anything made by the manufacturer that plugs in and does the same selection? If so, get a schematic and see how they did it. If not, I very much doubt you can hurt it, so long as it's just a pulled up pin.

There are several Motorola accessories of this type but I haven't had any luck in getting schematics. This model radio (Radius P50) seems to be a bit of an odball.

I'm relying on what I've read on another forum and the wisdom of AK. Once I get the connector in hand (ordered from Mouser) I'll measure voltate across a resistor just so I can be 100% sure I'm working against an internal pull-up resistor.

 

[adam-ads]

Two diodes in series worked out just fine. Thanks for the guidance.

-adam