Much simpler.
If bulb brightness, dim or bright is an issue, I'll try again.

 

Improved, not tested...........................

May not saturate below 10vdc.
Over 15 volts will start tuning on both.

 

And an led version tested fro 10 to 16vdc with 110 ohm.

I like this one. Think I'll convert one of my test lights.

 

Bulbs being lit aren't a problem.i I was surprised that there was voltage at tip of probe with nothing connected.electronics is a hobby.i am still learning.I will rewire tester using simpler circuit.
David

 

If using lamps, use 18v or 24v, to keep them dim in stand-by.

Some current has to flow when testing, so if you want to "test" to ground, some voltage must be available.

This can be minimized almost to zero. For the testing you are interested in, a fairly large current is good.

If it was too sensitive, just touching the probe would turn on indicator.

 

INWO,
I went up to shed and disassembled my tester.I really want to build your led tester.Could I trouble you for a parts list please?
David

 

How does the lamp model work?


R1 & R2 =100 (I used 110 ohm)
D1-D4 6.3v .5 watt Zener (I used 6.2v .4 watt)

That's about it.
Standard LEDs.
If you have others in mind, I can recalculate.

The reason my "as built" varies, is because I used what I had a lot of.
It's a pretty basic circuit.
Components in each leg can be in any order.
R1 & R2 control brightness.

Zeners control the max voltage input before leds glow while on standby.
It also interacts with the minimum and maximum voltage that can be sensed by probe.

Without exact specs from you, and keeping it simple, some trial and error might be in order.

Parts are so inexpensive that I would order a bunch of values surrounding those given.

 

Inwo.Thank you .The lamp model worked well.I then modified and fitted a test light with leds.
Works well.

 

Inwo.Thank you .The lamp model worked well.I then modified and fitted a test light with leds.
Works well.

Looks nice.

Is the one shown lamps or leds?

Are they shown dimly lit?

With he led method, leds will not be on until probing less than 4 volts or more than 8 volts. @12vdc

Dead band is centered on actual battery voltage however.

The low impedance of the lamp version may prove more useful.

 

The leds are bright .I only used leds and resistors.in order to purchase zeners , I have to travel to jaycar .over an hour away.i will get my friend to pick some up tomorrow.
Thank you
David

 

Be careful testing without zener. Max current will be exceeded.

What is your knowledge level of electronics?

Mine is limited to what I've learned hands on.

If you want an explanation of zener diode function, I will go into it using water analogy.

 

Basic knowledge of electronics.12 volt wiring on trailers, stock floats.House wiring.I wanted to be an electrician when I grew up.But I never grew up.Settled on being a builder by trade.I will take on board whatever knowledge you would care to share.I sit in my shed with power supply and a goodly amount of mini jumpers and experiment.
Thank you
David

 

A diode, as you may know is a one way, or "check" valve.

As no valve is perfect, it can't operate at unlimited flow or pressure without restriction or leakage. So it is with a basic diode.

An average silicon diode has a fairly constant forward voltage drop of about .6 volts.
Compare to a spring loaded check valve that requires ≈.6 psi in the forward direction. Pop-off pressure.

In either example there is little flow in the reverse direction. Only minor leakage.

Each also requires a time to operate. The diode function is electronic (near mass-less) and is much faster of course.

Also each can handle only so much flow "amps or current".

The water flow analogy differs here, in that while both will restrict forward flow, more, as the current increases, the diode will overheat. The friction heating is negligible in the water model.

Another important diode trait is its "zener" voltage.
This is the reverse voltage (pressure) that it can withstand without conducting in the reverse direction.

The valve analogy would be the strength of the reed in the check valve.
At a certain breakdown pressure the reed will no longer hold, and water will flow in reverse, with a pressure drop equal to the break down strength.

This is an important function that makes it useful as a regulator.

Diodes that are manufactured to optimize zener function are of course called zener diodes.

The zeners used in this circuit being back to back will drop the 6.3 zener v + the .6v forward v symmetrically. Add the 3v led and total voltage drop on each leg is ≈ 10volts total.

That leaves 3 volts extra (13 volt supply) which is limited to 30 ma by 100 ohms.

With no connection to probe, both sides are in series. Total diode drop is ≈ 20 volts. Enough so that no current will flow thru leds until the supply voltage exceeds total zener voltage.

I used lower zener values, so it tested ≈ 18 volts.

I reserve the right to edit, if I get caught in too many mistakes.
Generally I don't get it perfect the first try.