Free circuit schematics - wiparat.com

First bear it in mind that it is a single digit voltmeter which is 0-9 counts only on the positive side, that is it can measure +0 to +9V DC +/- 1V error. That may not be practical for the cost of the components above. It may be used as a toy logic probe. The [...]

The internal schematic diagram of the 4029 is quite complex, as it involves a gating structure that will allow counting either up or down, in binary or decimal, and with parallel inputs to preset a count. However, we can’t reach the internal circuitry in any case, so the functional diagram to the right is far [...]