Electronic – 7 segment LCD works on breadboard, not on stripboard

7segmentdisplaylcd

I am using a 7 segment 3 1/2 digits "raw" LCD (like this one) using 4 CD4543 controllers and a MSP430. I use one of MSP430's pins to create a 30Hz square wave that I feed into CD4543's phase input. Other MSP pins are used for the latch disable and binary inputs.

The LCD works fine on the breadboard and it's very stable (shows constant contrast over time).

However, when I tried moving the LCD (along with the 4543s and the MSP430) onto a stripboard (using more then 100 thin wires), the LCD kept fading away and behaved weirdly (human proximity or pressing on the wires behind the board would light it up or turn it off).

I checked and rechecked the wiring and could not find anything wrong.
I removed the LCD from the stripboard and it runs fine on the breadboard so it doesn't look like anything was wrong with the components.

What could be the problem that prevents my project from running on a stripboard?

Best Answer

Two obvious possibilities come to mind:

  1. You messed up the stripboard wiring. There is not much more we can help with that. You say there are over 100 wires, so the chance of a screwup, and then not catching it, is high. A real circuit board is the obvious solution.

  2. Your design left something floating, and it happened to work on the breadboard. Perhaps there was fortuitous leakage to ground or power in just the right place. It can take only a few nA of leakage to force a otherwise floating CMOS input high or low. It may also be that on the breadboard this floating input was more protected, but on the stripboard it couples better to power line hum or whatever.

Either way, it sounds like you have a floating input on the strip board, whether from a wiring error (point 1) or a design error (point 2). The fact that proximity of your hand changes the symptoms is a strong indication of this.

You have a bug in a circuit, so go diagnose it. The obvious thing to do is to look at waveforms on a scope to see what isn't right. I don't know what you expect us to do here.

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