Strange phenomenal?

[[email protected]]

Hi Guys,

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

Thanks
ywz

 

[Tam/WB2TT]

Hi Guys,

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

Thanks
ywz

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

 

[[email protected]]

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]

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.

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?