Strange phenomenal?

Question

Hi Guys,Recently I constructed a circuit using a voltage comparator, relay,transistor, some diodes and resistors. The output of this circuit is aLED. By having a specific range of input, the voltage comparator willenable the LED to be lit. LED is lit thru' the energizing of therelay. Theorectically, all calculations made are correct and thecircuit is properly soldered. However, the LED will not light upunless I TOUCH certain parts of the circuit (with my fingers or anymetallic objects). What can probably cause such an occurance?Thanksywz

Tam/WB2TT · Accepted Answer

My guess is that either:1. Circuit is oscillating. Add a few .1 uF bypass capacitors.2. Bad connection.3. You left unused CMOS inputs floating. Connect ALL unused inputs to GND or VCC.Tam

ydoubleuz · Answer

Tks. I resoldered everything and tied the floating pins.

The circuit I built is meant to measure resistance. It works perfectly for various loads(resistors values) but when i attempt to measure the resistance of some position sensors(Rx) it does not work. I measured the resistance of the Rx with a multimeter and replace that load with a resistor of identical value. Again the circuit works for resistors but not the position sensor circuit. Thinking it might be due to the input voltage, I reduced it to 2.7V but still it doesn't work.

Anthony Fremont · Answer

If what you are saying is that it doesn't work when you use long leads, then it is probably some kind of oscillation. Try putting a .1uF cap across the leads like Tam said. Can you show us a schematic?

Strange phenomenal?

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