I've received a board with the following circuit:
simulate this circuit – Schematic created using CircuitLab
The INPUT is a digital signal coming from a MCU, with HIGH value being 3.3V. Based on received INPUT, the circuit should operate in two states:
- STATE 1 : When INPUT is HIGH, OUTPUT1 is HIGH and OUTPUT2 is LOW
- STATE 2 : When INPUT is LOW, OUTPUT1 is LOW and OUTPUT2 is HIGH
OUTPUT HIGH = -7V
OUTPUT LOW = 0V
The circuit was tested and controlled by a MCU, worked fine in both states. However, when we tested another MCU, we started with STATE 1, and then driven the INPUT LOW, but it didn't switch to STATE2. In fact, it kept holding in STATE 1. And it only goes to STATE 2 when we touched the input channel with Voltmeter. We tried to add a pulldown circuit but that didn't solve the case.
Q1 & Q2 are BC516-D27Z
- Updated with the correct circuit schematic.
- Added OUTPUT HIGH/LOW values.
- Both MCUs are the same.