However, if I click the “Test” button, the LCD will light, but as soon as I release the button, everything goes off again.
I have tested the main board by looking at the +5V on pin 7 of the Atmega - but it turns out this is briefly just about 3.3V as long as I keep the button pressed, but drops to 0 as soon as I release it.
Further investigation of the VR2 (where the +5V is supposed to come from) shows that the input to the VR2 is already below +5V, so of course the output is too.
I take it the input to VR2, Vbat, comes from the collector of T1, and should be above 5V (the spec of VR2 suggests +7V) - but this is certainly not the case (when the button is pressed).
This is where I don’t know how to progress. Is T1 broken? Am I correct in thinking the collector should be >>+5V when the test button is pressed?
I have done a bit more testing: the voltage at the battery drops from 9v to just 5.5v when I press the switch - this battery is quite new (it actually measures a bit over 9v), so I wonder whether this drop makes sense - it seems too low to get the rest of the circuit to the required +5 it seems…
Is there anyone who has this working and can measure some voltages at crucial points for me?
PS: the voltage drops to 5.5v, even when I remove the ICs.
So, this turns out to be a problem with the standard chinese 9v batteries included in my electro kit. It just drops to 5v at a 5mA load. A switching power supply fixed this issue.
Next up, my LCD screen stays on - but nothing is displayed. Probably a solder problem with those tiny contacts for the LCD.
