Simple ways to convert low frequency pulses to voltage to drive analogue meter?

[Martin G7MRV]

Finding it quite tricky to think exactly how to ask this! Essentially, im still playing with simple Geiger-Muller circuits, and am now looking at simple ways to turn the detector into a counter!

Clearly, there must be a reasonably simple way to do this - vintage Geiger counters from the '50s did it! But ive had remarkably poor success in finding the circuit diagrams of vintage meters to see how it was done.

I'd like to avoid ICs if possible for the time being (another project is using the ultra-modern microcontroller approach!). To my mind simply feeding the pulses to the meter through various high value capacitors would 'smooth' out the pulses at higher rates, but would be rubbish at low rates.

So im interested in any 'simple' ways to take pulses in the frequency range of less than a few Hz up to maybe a couple of kHz (dont mind if it has to be done in 'ranges' of course), to create a variable voltage sufficient to give a reasonably steady indication on a typical moving coil meter?

Of course, circuit diagrams of vintage Geiger counters and their meter circuits would be great!

Looking forward to seeing some ideas!
Martin

[Scimitar]

I know that you don't want it, but that job is perfect for a simple Arduino interface.

[Ambientnoise]

The simple way would be an charge pump comprising a series Diode feeding a Capacitor/parallel R to ground. Effectively this peak detects and smooths the pulses into a voltage. The voltage decays at a rate depending on the R and load from the meter. The input pulses need to have a stable amplitude and width or you will need some extra circuitry. Try a search for frequency to voltage converter. You are correct that range switching maybe required. HTH.

Ken

[Martin G7MRV]

Just a note - Ive now got diagrams for the Victoreen CDV-700 models 6a and 6b. It seems much of my problem finding diagrams was due to the works IT systems!

Im still having trouble making sense of them though! Partly due to the PNP circuitry - im not used to working 'the wrong way up', plus some strange transistor/transformer arrangements. These old models also use anode pulse detection, which is rather frowned upon these days.

[Martin G7MRV]

Originally Posted by Ambientnoise

The simple way would be an charge pump comprising a series Diode feeding a Capacitor/parallel R to ground. Effectively this peak detects and smooths the pulses into a voltage. The voltage decays at a rate depending on the R and load from the meter. The input pulses need to have a stable amplitude and width or you will need some extra circuitry. Try a search for frequency to voltage converter. You are correct that range switching maybe required. HTH.

Ken

Looking at the Victoreen diagrams ive now managed to find, I think this is the way it works. I can see the diode/capacitor pair on the diagram.

Trouble with a general search for 'geiger counter circuits' is that you get flooded with either just detectors ('clickers') or Arduino based circuits!

[Terry_VK5TM]

Some simple circuits here:-

http://www.techlib.com/science/geiger.html

Search term used was - geiger counter meter drive schematic

[Argus25]

You can solve all of your problems for this for less than $10 if you go for a pre-made digital meter module.

But you are very non specific when you say you want a "counter".

You can buy meter modules which give a cumulative count. With the input pulse the display just adds 1 each time.

Or you might want instead not a counter, but a "frequency meter" even though you said counter.

In this case you can easily get an RPM digital meter (which shows counts per minute) or a typical frequency meter that shows counts per second. These have a time domain constant of course and decay to 0 with zero pulses.

These cheap digital meter modules are all over ebay.

(I used both an RPM meter module and a totalizer count module to monitor the chuck on my lathe, very helpful winding coils and cost less than $20 from China of course)

Picture of a plastic box attached, with two cheap LED modules mounted in it, one is a totalizer the other a count per minute (RPM) meter

[Martin G7MRV]

Originally Posted by Argus25

You can solve all of your problems for this for less than $10 if you go for a pre-made digital meter module.

But you are very non specific when you say you want a "counter".

No, I was very specific - I want to avoid ICs on this simple project, and am running a parallel project using PIC microcontrollers. This was mentioned in my post! As was the fact that I wish to drive an analogue, moving coil meter. Clearly as well I am looking to recreate a 'vintage' type counter circuit.

It looks like the diode-capacitor charge pump is the way to go. I dont mind playing around with range resistors etc, its all part of the fun of playing with these circuits. My pulse heights are pretty constant, pulse width i'll have to check whether it would need stretching or not.

Searching for charge pump on the 'net predictably trows up mostly SMPSU and HV circuits!

[Martin G7MRV]

Originally Posted by Terry_VK5TM

Some simple circuits here:-

http://www.techlib.com/science/geiger.html

Search term used was - geiger counter meter drive schematic

Hi Terry,

My HV circuits are mostly derived from circuits on that site! But unfortunately only one has a meter and is an actual 'Geiger counter', the rest are just detectors, and that one uses an IC! I will be trying one of the HV feedback techniques on this version though.

[Hengist]

Hello Martin
I had the same requirement recently. I ended up with a 2 transistor monostable to make the pulses fixed width. The pulses then charge up a capacitor which feeds a 100 microamp meter. I included a couple of preset resistors to enable the circuit to be calibrated from a pulse source of known frequency.

[Neutrino]

Attached is the circuit diagram for a Ratemeter Type RM5 divided into 4 for clarity.

The overload detector gives an audible alarm and prevents the indication from falling off at high count rates. Otherwise at high count rates the amplitude of the pulses decreases and the dead time causes the pulses to merge giving a false low reading when you should be running for your life!

Instead of range switching, it has three diode pumps to give a 4 decade ‘logarithmic’ scale from 0.5 to 5,000 counts per second. Each diode pump has a different value smoothing capacitor. For anyone interested in a project, a transistor can be the basis of a neat logarithmic amplifier although temperature compensation is needed for better accuracy.

Ratemeters Type RM5 sometimes appear on ebay.

David

[Martin G7MRV]

Originally Posted by Hengist

Hello Martin
I had the same requirement recently. I ended up with a 2 transistor monostable to make the pulses fixed width. The pulses then charge up a capacitor which feeds a 100 microamp meter. I included a couple of preset resistors to enable the circuit to be calibrated from a pulse source of known frequency.

I think that might be the case here. Another of these projects is using 555 timers, with one as a monostable. This is why I want this particular project (using a sub-miniature G-M tube) to not use any ICs!

Ive six G-M tubes to play with! So Im working on two as 'detectors' (clickety-click), two as traditional 'counters' using analogue meters (one IC based one transistor) one Arduino or PIC based, and one, with a very delicate alpha tube, as a 'lab' instrument. My tubes range from 20mm long to 25cm long!

This particular, non-IC version, will use a tiny tube and be a very compact little 'pocket' Geiger counter! The other tiny tube will be the 555 timer based unit, also a pocket device, but just a detector.

[Radio Wrangler]

LM2917

David

[julie_m]

What you want is a monostable, which will give a fixed width pulse when triggered; and then to feed the output of the monostable to a moving-coil meter. The faster the pulses are coming, the more time the output will spend in the "on" state, and so the higher the meter will read.

You should be able to build this without any ICs -- probably even using a dual triode valve!

[Argus25]

Originally Posted by Martin G7MRV

I want to avoid ICs on this simple projec

Sorry, I had gathered that you simply didn't want to design/make/program one yourself. The meter modules I suggested are pre-made, you don't even need to go inside them and treat them as a black box. Most of these do have PIC micros inside them these days. The older ones had IC's from Intersil.

For an analog meter, the simplest I can think of would be a relatively small capacitance charged by a diode, so it fully charges by the width of your pulse, with a very slow discharge, perhaps a 10M parallel resistor, and that connected to the gate of a mosfet (or triode) and the meter simply in the drain circuit or anode with a parallel capacitor across the meter to integrate the widely spaced pulses. So maybe 5 or 6 parts. I would guess the meter's face would have to be calibrated experimentally. You could also make sure the capacitor charges to the same voltage each time, say 10V with a parallel zener diode and a small logic level mosfet or similar like a BS270 should work. Maybe start with a 1000pF capacitor in the gate and a 10uF across the meter.

[G6Tanuki]

I'm thinking that this is - deep down - really rather like what an old-style car revcounter/tachometer does, using the 'digital' on-off pulses from the contact-breaker to drive an analog meter.

Joe Lucas, the "Prince of Darkness", managed to do this in the 60s using a couple of Germanium transistors, a couple of diodes, and a handful of resistors/capacitors.

In the 60s and 70s the electronic-hobby magazines were full of 'revcounter' projects - could you perhaps repurpose one of these?

[Personally, I'd go for a 555 as a monostable to debounce/convert the pulses from your GM-tube into something with limited amplitude/duration, followed by a simple diode-doubler into a R-C network feeding a 100uA meter]

[Martin G7MRV]

Originally Posted by Argus25

Sorry, I had gathered that you simply didn't want to design/make/program one yourself. The meter modules I suggested are pre-made, you don't even need to go inside them and treat them as a black box. Most of these do have PIC micros inside them these days. The older ones had IC's from Intersil.

Dont get me wrong, for many purposes, especially when I just need to 'get it working', im all over the low cost ready-made modules! (why muck about making a TTL-RS232 interface or a Buck converter when they can be had ready and working for a quid?), but in this case ive time to play and so want to try ideas out, and learn how the system is working and how to make it do things!

Theres much more fun to be had in trying out and modifying a circuit idea to get something working than there is an a ready made module! Over the weekend i'll breadboard another HV circuit (trying a different inductor set-up than last time) and try out a charge pump meter circuit.

[Hengist]

Hello Martin
If you haven't sorted out a counting circuit yet, you may be interested in a video showing my solution mentioned earlier. Alternatively, if you've come up with something better (but still simple) I'd be very interested!
https://www.youtube.com/watch?v=8zM1HG9Agzk

[Martin G7MRV]

I was meant to work on it over the weekend and today, but other priorities got in the way! Hopefully mid-week i'll have time to try the circuits out

[mole42uk]

Did you have a look at the PPM type of quasi-peak detector circuits? The BBC use them for sound level measurement, analogue sound peaks being converted to drive an analoue panel meter. There are circuits available for early transistorised ones and later op-amp based ones.