Interesting Sensors

[Sebastian]

I have to build a PIC µC based "multimeter" at school. The sensor part has to be 1x1 in and is connected to 5 I/O pins of the controller and two of them can be used with the integrated 10bit adc. The supply voltage is 5V so i can not really use more than that.

So I am looking for interesting sensors or other stuff i could measure. I was thinking about a coax cable length meter that applies a fast rising edge to a open ended coax and then measures the reflection but the PIC is clocked at 4MHz so one instruction cycle is 1µs which means I can not measure short enough times to do that. Of course I could jut make a voltmeter, currentmeter, thermometer or something like that, but I would like to do something more challenging so I am looking for ideas. Keep in mind I only have 1 by 1 inch of double sided PCB .

[Rudane]

With the coax you'd need more than 100 meters to get your time of flight signal. How about a photodiode that measures the amount of ambient light and turns on an LED when the amount of ambient light is too low. This would also require some level of hysteresis so the LED didn't turn on and off sporadically near the threshold value. It might also be tricky to remove the effect of the LED's light on the photodiode. Maybe you'd have to consider physical placement options.

[SeanB]

Use one pin to drive an IR led with 200mA pulses ( 1% duty cycle) and use the 2 inputs to read 2 photodiodes, and make a proximity sensor that is sensitive to direction of approach. The remaining pins can be used to drive a LED with PWM to give an indication of range - off when far growing bright when something approaches the board. The LED will need good decoupling to keep the noise injected into the ADC input low.