I am designing a cable tester for scope probes, DMM leads etc. It will basically be a small box with a green 'cable good' indicator and a 'red' cable bad indicator (LEDs) and will run off of a 9V battery. A small PIC microcontroller will handle sensing when a cable is attached and then performing a pass/fail test.
The power source has the following requirements:
1. No on/off switch, the cable being plugged in will automatically turn on power to the microcontroller via a 3.3V LDO.
2. When the battery is replaced, the unit will also power on automatically and perform a 'battery good' test.
3. After the microcontroller performs the cable test, it will remain on for about 30 seconds after the user removes the cable being tested. The microcontroller will turn itself off.
I have the following sub-circuit designed to handle the above requirements. Note: I am not using the shown 3.3V LDO, but it will be similar with an 'Enable' input like this one:
Circuit notes:
- When the battery is connected, C1 starts to charge, turning on M1 which then enables the 3.3V LDO that provides power to the microcontroller. (This is a sub circuit only, the microcontroller is not shown).
- D3 then latches power on, by placing the 3.3V output to M1 holding it on.
- Cable_In1 and Cable_In2 are typically the 'ground lead' of the cable being tested. These are shorted together when the cable is connected turning on M1 which then latches power on to the microcontroller. This turns off M2 which provides a 'Cable_Connected' digital input to the microcontroller.
- R6 and C4 are not part of the circuit but here are simply emulating the microcontroller 'MICRO_SUICIDE' digital output.
- M1 and M2 are not the final parts. I simply selected them being they are part of LTSpice at the moment.
- When the microcontroller sets 'MICRO_SUICIDE' high, Q1 turns on via D4, which in turn turns on Q2. This latches Q1 and Q2 on which turns off power to the LDO. Which then allows, C1 to charge and turns off M1 resetting the circuit.
- D1 and D2 protect the circuit from a battery being plugged in 'backwards'
- C2 just does some filtering
- Filtering for the LDO is not included here but would be on the final circuit.
I am considering:
- Replacing Q1, Q2, D4, R9, R8, and R7 with a small SCR to reduce parts count.
I have also included the LTSpice circuit.
Here is my question:
This seems rather complicated to accomplish my requirements. Does anyone know of a simpler way to accomplish this without so many parts?
I also just noticed that M2's gate is connected to one of the outside connections. This should probably be protected from ESD damage from an ESD source, what is a good device to use for that?
Thanks!
The power source has the following requirements:
1. No on/off switch, the cable being plugged in will automatically turn on power to the microcontroller via a 3.3V LDO.
2. When the battery is replaced, the unit will also power on automatically and perform a 'battery good' test.
3. After the microcontroller performs the cable test, it will remain on for about 30 seconds after the user removes the cable being tested. The microcontroller will turn itself off.
I have the following sub-circuit designed to handle the above requirements. Note: I am not using the shown 3.3V LDO, but it will be similar with an 'Enable' input like this one:
Circuit notes:
- When the battery is connected, C1 starts to charge, turning on M1 which then enables the 3.3V LDO that provides power to the microcontroller. (This is a sub circuit only, the microcontroller is not shown).
- D3 then latches power on, by placing the 3.3V output to M1 holding it on.
- Cable_In1 and Cable_In2 are typically the 'ground lead' of the cable being tested. These are shorted together when the cable is connected turning on M1 which then latches power on to the microcontroller. This turns off M2 which provides a 'Cable_Connected' digital input to the microcontroller.
- R6 and C4 are not part of the circuit but here are simply emulating the microcontroller 'MICRO_SUICIDE' digital output.
- M1 and M2 are not the final parts. I simply selected them being they are part of LTSpice at the moment.
- When the microcontroller sets 'MICRO_SUICIDE' high, Q1 turns on via D4, which in turn turns on Q2. This latches Q1 and Q2 on which turns off power to the LDO. Which then allows, C1 to charge and turns off M1 resetting the circuit.
- D1 and D2 protect the circuit from a battery being plugged in 'backwards'
- C2 just does some filtering
- Filtering for the LDO is not included here but would be on the final circuit.
I am considering:
- Replacing Q1, Q2, D4, R9, R8, and R7 with a small SCR to reduce parts count.
I have also included the LTSpice circuit.
Here is my question:
This seems rather complicated to accomplish my requirements. Does anyone know of a simpler way to accomplish this without so many parts?
I also just noticed that M2's gate is connected to one of the outside connections. This should probably be protected from ESD damage from an ESD source, what is a good device to use for that?
Thanks!
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