The Curious C-beeper

[dave_n_t]

I'd been tempted for some time by Charles Wenzel's "Curious C-beeper and finally have put solder to board.

If you search the internet for "Curious C-beeper", you'll find a loads of examples, and a couple of variations of circuit. I thought I'd stick largely to the original. (OK. I'll admit - I hadn't seen all the variants before I decided to make the original )

First change I made, though, was to swap polarity. I had some NPN darlingtons, but no PNP ones (yes, I could have made my own, but that would have been about 10% more effort!). Mindful of the dire warnings about substituting transistors, I measured the hFE of candidate transistors using my all-singing all-dancing cheap component tester. I ended up (FWIW) with an FST238 darlington; a BC549C and an SPS5123. Of these, only the middle one had a recognisable type number, the others having been rescued/acquired over the years.

My first attempt was on 0.1" veroboard, in the hope of being able to pack things in quite tightly. It worked, but was somewhat unstable. (In retrospect, maybe selecting for extremely high gain wasn't such a good idea!). I guessed I was having problems with some, or all, of stray capacitance between components, and via tracks; leaky board (years of deposits of grime between the tracks; flux soaking into the srbp); copper or solder whiskers.

So I rebuilt it dead-bug style on a piece of double-sided PCB. This produced a stable circuit with the sensitivity that Chuck advertised. I decided to use the PCB as the basis for the housing, and my favourite hot glue to fix a few things in place. I also put in an on-off switch, as a circuit fault (or, for example, the probe touching a large piece of metal) might lead to constant triggering and battery drain. On finishing the 'case', I found that I couldn't reduce the variable capacitor's capacitance sufficiently to stop the buzzing; in desperation, I unsoldered one leg and put a 4.7pF capacitor in series. That resulted in sufficient capacitance to give the sensitivity I wanted without buzzing 'at rest'.

My favourite party trick is to touch the probe on a piece of copper foil about 10cm by 2cm; nothing happens. Then I put a polythene bag on top + press down with a thumb (holding the beeper in either hand...). That is sufficient capacitance to trigger it.

I had a fun few hours knocking it together, although I'm not convinced I'll find it as indespensible as others have claimed!


Pictures of the build-up to the final thing:

[Andrew2]

Fascinating! I may build one just for the hell of it. I've got a certain fondness for novelty stuff like this, probably stemming from my schooldays, when my friends and I made small noise makers to get the teachers wound up.
A while ago I built a subsonic detector as a matching 'bookend' to my ultrasonic bat detector. It's the same thing (more or less) with an electret mic with op-amp low-pass filters, a mixer /oscillator (running around 1KHz) and a small output amp. It makes ghostly piping sounds whenever there's a truck approaching or the neighbours open or close a door. I like to leave it on just to see what's going on, but it annoys the heck out of my wife!

[Radio Wrangler]

A suitable sound generator for a replica of the machine which goes 'PING"?

David

[dave_n_t]

Originally Posted by Andrew2

A while ago I built a subsonic detector.... It makes ghostly piping sounds .... it annoys the heck out of my wife!

Andrew,
Thanks!

My equivalent of your ultrasonic set-up was a small box (powered by a lithium cell) and a 40106-based circuit arranged to beep about 10 times every 3 minutes or so. I really built it to explore the low current consumption of the 40106 (6 schmitt gates in one chip). I ended up putting it on top of the glass cabinet in the dining room (which was largely only used for visitors). One of my wife's visitors commented that we seemed to have a lot of lorries reversing near the house, which she couln't understand, because she didn't see any.....

dave

[Refugee]

That would not happen now that lorries are fitted with sounders that make such a horrible noise that nobody would stay behind them.

[Herald1360]

Originally Posted by Radio Wrangler

A suitable sound generator for a replica of the machine which goes 'PING"?

It might be better for detecting octiron......

[Al (astral highway)]

Nice one, Dave!!

An interesting circuit from the imaginative mind of Charles Wenzel! Techlib is great.

I've shared a modulator design of CW's here before. I like the appropriation of an Altoids tin, although I wish I had kept the only one I've ever come across.

Thanks for sharing!

[Graham G3ZVT]

I've never quite understood the Altoids thing. My projects used Ogden's St. Bruno pipe tobacco tins.

[G6Tanuki]

I remember making something similar many years ago using two strings of cascaded 4017 CMOS chips along with some diodes and R-C timeconstants to form an AND-gate; each string was clocked from a different oscillator so they ran at slightly different frequencies: only when a specific pin on a 4017 on each chain went high did the diodes/R-C networks pass a brief 'enable' pulse to the third oscillator which produced the 'chirp' through one of the little flat piezo-sounders. It'd chirp randomly with intervals between 30 seconds and several hours!

Current-consumption from a pair of D-cells was about 20 Microamps. I built the whole thing into a foot-long length of white plastic water-pipe which meant it was easy to conceal (up behind the sink where there are plenty of similar-looking water-pipes) in my victim's kitchen.

[dave_n_t]

My projects used Ogden's St. Bruno pipe tobacco tins.

I was going to try to use a Nipits tin (approx 2.25"x1.75"x0.5") but couldn't quite squeeze it all in....


(And I remember using snuff 'tin' quite some years ago for the first ZN414 receiver published).


dave

[PatMorgan]

There was a circuit /project in PE sometime in the late 70's of a VCO and waveform modifier that my mate spent litterally months building and finally delivered this to me as a 'late birthday present' It was used as a 'siren' in my Opel Manta, as well as a PA - via the period 'Eagle' waterproof horn in engine compartment. The fun I had with the setup doing nights and weekend callouts all over London and the South-East. You would get 'lockedup' if you did that thesesdays! As G6Tanuki said, the ramdom noises/timebase/ramps causes havoc sometimes in traffic-jams for the same childish ...but everso-funny reasons. Tuning in the actual 'siren' pattern (which was a bit hit and miss) would cause the seas to part....