This is a conduction tester to check the connection on the built proto-board. Normally a circuit tester is used to check the conduction. But it is not suitable to check if wired or not in the circuit board with mounted parts because the semiconductor parts in the circuit become conductive under test voltage. Also the test voltage and current applied to the circuit can damage electrically sensitive parts. The in-circuit conduction tester can check pure conduction (wired or not) without those problem.
There are some requirements and considerations for the in-circuit conduction tester described below.
Right image shows the in-circuit conduction tester (ICT) that designed on the above requierments. I had built two ICTs past days and this is the third design. In this design, a microcontroller is used to simplify the circuit diagram. The test voltage is generated with a voltage divider, R2 and R3. The open voltage between test probes is 500mV and short current is 100μA. When a conductive is probed, the output voltage will dip and it is captured with an A-D converter.
The power switch is omitted and it is managed by sleep function of the microcontroller. The measured supply current was 400μA at power-on and 1μA at power-off. The battery life will reach 500 hours when a CR2032 coin cell is used. To hold the coin cell on the board with small foot print, a spring wire (0.3mm phosphor bronze wire) is useful.
The generic test lead is not that useful for conduction test on the circuit board so that I modified the test leads and attached test probe tips. It can probe 0.5mm pitch ICs. The circuit board is enclosed into a Takachi SW-53 plastic case in size of 36(W) x 53(D) x 11(H). It is useful size for simple gadgets.
The firmware processes only a siple loop that reads A-D value and sounds a piezo buzzer. The beep tone varies depends on the resistance. When the resistance is larger than 5 ohms, the frequency start to lower and stopped at 45 ohms. Therefore it can detect a contact trouble by the beep tone.
At power on (wakeup from sleep state), it tests output voltage and indicates the low battery status (blinking LED) when battery voltage is below 2.4V. When no conduction is detected for 5 minutes, it enters power off state.