I'm stumped and frustrated. I made a fairly simple circuit to sense water levels in a chicken waterer. The goal was that it would light Red LEDs when near empty, one Green LED when partially full, and three Green LEDs when full. Here is a simulation of the circuit I am using. (Requires Java and uses a free circuit simulator)
Notes:
In Simulation, it works.
On the solderless breadboard, it worked.
Soldered together in final? The green LEDs all work as expected, but the Red LEDs stay lit continually.
Debugging steps I've taken: (None of these produced results on identifying what the problem is)
Notes:
- The transistors are 2N5551
- The LEDs are from my parts bin, so I had to guess their values, but my multimeter shows the amperages are what my simulation predicts.
- Two switches in the simulation simulate how the sensors behave when they sense water at various levels.
- The power source is AC because the sensors pass current through liquid, and the contacts will corrode rapidly with DC and require frequent cleaning. I've read that using AC vs. DC make huge differences in cleaning intervals.
- The circuit began with this design, and was heavily modified by me to suit my purposes.
- Added the RED light
- Changed voltage to suit my power source and my LEDs
- Moved resitors for limiting current to LEDs to physically next to the LEDs, so I would quit burning up LEDs while testing.
In Simulation, it works.
On the solderless breadboard, it worked.
Soldered together in final? The green LEDs all work as expected, but the Red LEDs stay lit continually.
Debugging steps I've taken: (None of these produced results on identifying what the problem is)
- All three transistors (2N5551s) work
- All resistances are correct.
- Confirmed source voltage
- Checked Voltage across LEDs
- Goes up/down appropriately for the green LEDs
- Stays constant across the Red LEDs (roughly 8.5V, which is right for ON)
- Increase the base current of the transistor by decreasing the resistor values to try to make the transistor "open wider"
- Searched the PCB for any solder bridges, and hunted for shorts with a multi-meter, even de-soldered and re-soldered several component to look for issues.