A square wave test signal generator for testing electronic projects (3)
HOW DO I BUILD IT?
Here's the real "icky" part and it depends on what you can scrounge. Even if you puchased the above parts new you should not have spent $1. What we do next depends entirely on how you want to package your square wave signal generator.
As it is going to be a "service" tool we want to house it in someting hand held. If you can't buy anything else suitable, consider scrounging a piece of PVC plumbing tube or even electrical conduit, say about 1 1/4" (32mm) diameter (must be able to accomodate 9V battery with snap attached), cut to a suitable length of about 4" (100 mm). These dimensions are not sacred (from memory my original one was 2" - 50 mm tube) as long as we have space for the battery, our circuit board and, these other components AND it's comfortable in your hand.
To this we are going to add a probe sharpened to a point at one end. Also we are going to add a red led (light emitting diode) and a push button switch. See figure 3
Fig 3 multivbrator / square wave signal generator diagram
From this diagram you can see we have a piece of PVC tube which will conveniently house a 9V battery with it's battery snap (clip) attached. We will have a piece of Vero or perf board suitably sized to fit. Also we have glued in our red led (after drilling a suitable size hole) and similarly we have fitted our push button switch. This probe we might have made from a discarded screwdriver which has had the head damaged but we have now ground it down to a pointy end and preferably covered with some sort of insulation such as heat-shrink tubing, that is except for the pointy tip.
Hey! - y'all have to improvise here folks.
Now the whole idea of this is we have a giant "Biro" or ballpoint pen which fits comfortably in our hand, allows us to use our forefinger to press the push button and "believe it or not", on mine, the red led provided some illumination in the deep recesses of radios.
Also with mine (the original one that's gone to the sky) I got cunning and found if I used a hole saw and drilled a hole in some scrap material slightly larger in diameter than the inner diameter of the PVC tube I had a neat end plug to glue into the PVC tube to house the old screwdriver. I actually used 1" thick scrap timber as I found this gave me some rigidity in holding the probe.
FINAL MUTIVIBRATOR CIRCUIT DIAGRAM
Fig 4 multivbrator / square wave signal generator - final schematic diagram
From this you can see where the black and red leads from the battery snap connect to. Doesn't matter which way your 0.01 uF capacitors connect (buy cheap 0.1" lead spacing types) nor do the resistor connections matter.
Resistor colour codes:
39K - if 4 colours - has Orange|White|Orange|Gold = 39K 5%
39K - if 5 colours - has Orange|White|Black|Orange|Red = 39K 2%
330R - if 4 colours - has Orange|Orange|Black|Gold = 330R 5%
330R - if 5 colours - has Orange|Orange|Black|Black|Red = 330R 2% ???? Check!
Check your resistor values with your multimeter.
The pushbutton should be a "normally off" type meaning it ONLY connects when you push the button.
Now the transistors and the led, the second hardest part. If you don't know the leads for certain, then wherever you buy these components from, have the sales clerk identify the leads for you. The transistors have an emitter (E), base (B) and a collector (C). The led has a cathode (k) and an anode (a).
Connections are shown on the final schematic. If you can't have the leads identified from where you purchase - DON'T BUY THEM.