How to prevent voltage glimpses on BS2 pins on power-up?

CuriousOne

Hello.

I have a simple motion sensor setup using basic stamp, so when motion is detected, the stamp enables the flash and camera, to take photo. The flash is connected to BS2 via 2N3904 transistor, which, via 470 ohm, is connected to BS2 PIN. The problem is, when I turn on the power, a small glimpse of voltage occurs on BS2 output, which is enough to trigger the flash. How can I solve this?

Mike Green

First of all, the transistor is a current-operated device, not a voltage-operated device, so you need significant current flowing out of the BS2 pin to turn the transistor on. The BS2 I/O pin starts as an input pin on power up which is a high impedance and can't supply current to the transistor. There may be some other source of noise that might provide enough current to fire the flash. You could try putting a 1K or higher resistor from the transistor base to ground to try to keep the transistor off during power up. If that doesn't work, you'll need to do some sleuthing with a meter or scope to find out what's really triggering the flash.