Question regarding the output Pins

Aaron R

So I don't have a voltmeter, and its likely someone on the forum has the an answer for my question.

I have a small circuit that works just fine when its v+ is connected to VDD on my board, but when I switch the wire from VDD to P15 and set it high, the circuit does not work. The only problem I can see is that the Voltage on the output pins is less the VDD on the board. can someone verify this?

I tested the pin with an LED.

Thanks in advance.

Mike Green

The voltage on the output pins is indeed slightly lower than Vdd. There's a voltage drop across the output transistor of about 0.7V, but this varies somewhat with the amount of current drawn. The lowest voltage produced also involves a voltage drop of about 0.6V.

A Stamp I/O pin can only supply a little current, something on the order of 25mA. If your circuit requires more than that, the Stamp can't supply it.
Typically, a higher current would be switched by some kind of power transistor which the Stamp can control. There are all sorts of examples of this including Nuts and Volts Column #6 which you can find under the Resources tab on the main Parallax webpage.