Maximum Trigger voltage

[time2dive]

I have a break beam kit that normally runs a buzzer.  It outputs 9v when the beam is broken, 0v when it is not.  Is that voltage too high for a Prop1 trigger input.  I would rather ask first then wire it up and burn out a Prop1.

Tim

[bsnut]

Tim,

I am glad you posted this question. Because you would've "blue smoked" the Prop-1 the I/O pin with too much voltage.

The Prop-1 which is a Basic Stamp 1 chip works on 5VDC and 9VDC would've killed the I/O pin that you were planning connecting to. You need to make an circuit to allow you to use this break beam at the voltage that it operates at. If this break beam has relay contacts to use then you can connect up to the Prop-1 I/O pin at "R" and "W", which this would be ok to do and you don't kill the I/O pin. Now, if you don't have these relay contacts on your break beam then you will need to make interface using a optoisolator to keep from killing the I/O pin.     

[JonnyMac]

If you insert a 22K resistor inline with the pin that will limit the current and protect the Prop-1.

9v -----/\/\/\/----- Px.W
            22K

I've done this many times.

[time2dive]

Thanks....saves me doing the math and make a voltage divider

Tim

[bsnut]

Thanks Jon, I should've tough of that way. I am just optoisolator fan, since they can be used for higher voltages. Of course this is the same method that is used with Propeller to protect its pins from high voltage, but you using it with the Basic Stamp.

[JonnyMac]

Actually, I do the same thing with the Propeller all the time; this lets me interface the 3.3v Propeller with 5v devices (we do this on the AP-16+ and HC-8+).

Here's the key: get the spec sheet for the target processor and find out what the MAXIMUM current through the pin protection diodes is.  Use this current and the forward voltage of the diode to determine the series resistor.  Since I do so much Propeller work I'll use it as the example:

Imax = 0.5mA
Vfwd = 0.3v

Given a 5v circuit into the Propeller Rseries works out to  (5 - 3.6) / 0.0005 = 2800 (I always use 3.3K as this is the next highest standard resistor size).

[time2dive]

I tried a 22k resistor and it did not appear to lower the voltage.    Two 5k resistors as a voltage divider works just fine.   White lead to 9v black lead at the center of the voltage divider gives me a 5v trigger.  The magic smoke stayed in the Prop1.

[JonnyMac]

That works, too, and is actually simpler.  Nicely done.