Some times I have problems geting my head around the simplest of switching 
circuits.  For a long time I've used sonalerts or similar radio shack 
beeper as continuity testers.  I wanted one that would be a continuity 
tester as well as a "beeping test light" that would beep if dc voltage was 
present.  This simple little circuit will do just that.  In continuity 
mode, it will check fuses, diodes, some capacitors and so forth.  In the 
voltage test mode, it will tell you if voltage is there and what the 
polarity is.  The neat thing about these little beepers if if you hook 
them up backwards, nothing happens, they just don't beep.  The ones I use 
are good from about 1.5 volts up to 20 volts.

Parts and circuit.  All you need is a couple test leads, plus and minus, a 
sonalert or similar beeper, a double pole double throw switch with center 
off, a xmall box,  and a nine volt battery and clip.

Picture the switch terminals as a braille cell.  Left terminals from top 
down are 1 2 3 and right column from top down are numbered 4 5 and 6.  The 
commons are of course 2 and 5.  In one position, 2 is connected to 1 and 5 
is connected to 4.  The other way, 2 is connected to 3 and 5 is connected 
to 6.  Here is the simple little circuit.

Common terminal 2 goes to positive test lead.  Common terminal 5 goes to 
negative test lead.  Terminal 1 goes to the positive side  of the 
sonalertand a jumper wire to terminal six.  Terminal 4 goes to negative of 
the sonalert and to the negative of the 9 volt battery.  Terminal 3 goes 
to the positive of the battery.

That is it.  In one position, 2 is connected to 1 and 5 is connected to 4. 
This simply connects the test leads to the sonalert to test for dc 
voltage, on a power supply or in a car.  The 9 volt ground is on terminal 
4 which doesn't matter.

In the other position positive test lead on 2 is connected to 3 which is 
the plus of the 9 volt battery.  The negative test lead on 5 is connected 
to 6 which is jumpered to 1 which is the positive of the sonalert. 
negative side of the sonalert is already grounded on terminal 4.  This 
gives you a continuity tester to check fuses or diodes and since you know 
which lead is plus, you know the polarity.  The same is true when checking 
voltage.  If plus is to plus and negative to ground the thing beeps.  If 
it is backwards, no beep so you know plus and minus dc volts.  It is 
73
Butch
WA0VJR
Node 3148
Wallace, ks.


really a handy little box to have around.