Electronic test light = "Electronic "Probe"
On page "Tips and Tricks " of my website I've already described how to make a test light. This did not use any electronic compontents, only 2 leds. Main disadvantages were that short pulses could not be seen, and the current drain on the mesured signal was too high...
With this electronic test light these disadvantages have disappeared, and we have an easy test instrument that I always use when troubleshooting and following signals.
The test light which I present here uses 3 leds to indicate the different signals. A red led for logical 1, a green for logical 0, and a yellow for pulses. These pulses can be short or long, it doesn't matter, a one-shot amplifies them so they will result in a clearly visible led signal.
In general it is far more easy to follow and control signals on the boards with the electronic probe.
The first ic 7403 is used as buffer, so the extra draw on the signal to measure is only one logical gate. The output of this gate is connected with the input of the next gate, and on both outputs are leds connected for logical 0 and 1. The signal also enters ic 74121 on pin 5 this "one shot " will prolong every change (pulse going up or down) into a signal which is long enough to be noticed on the output (pin 6) connected to the yellow led..
I've mouted everything to a piece of pcb. The pictures explain the rest..
Here a more professional schematic design ....
Three led's , green , red en yellow, three connectors: red, blue and black, three resistors of 1,2Kohm, and one of 4,7Kohm , a condo of 2,2µF, a universal print, and ic's 7403 and 74121, with sockets.
The important parts mounted:
When using this, sometimes you'll notice the 3 leds light at the same time, this means an alternating signal with about the same ratio between the positive and negative part of the signal... ( Fig a)
When i.e. the red led and the yellow go on at the same time, you have a positive signal with negative pulses, which are so short that the green led does not light. This is why we prolong the pulses so they become visible on the yellow led... (fig c)
In the opposite case, you have short positive pulses, the yellow and green led will light...(fig b)
Such short pulses, which can only be detected using this kind of probe, are most of the time selection-signals.
I leave it to your imagination to put this into a nice box or other housing..
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