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adibrook · 23rd Oct 2006, 6:35 pm

Today i took a little trip to the local 99p store to buy some of those useless door alarms. They are magnetically activated beepers that go on your door and suppoedly squeal when the door is opened and the magnet moves away. Although most of the time they just squeal all the time even when the magnet is by them.

But...i just seem them as a cheap source of piezo drivers. Where else can you get 3 piezo drivers for 99p?

Today however i bothered to look at the circuitry instead of just rippign it out. It turns out that its a miniture HV generator to power the piezo driver.

Since piezo drivers are high impeadence i guess it makes more sence to run them at high voltage.

So i connected a scope to the piezo driver. And...what do you know...it was hitting 70v!

It seems that the little IC in the alarm produces a 4KHz pulse which is FM modulated to about 2Hz or so to give the distinctive alarm sound. This is then stepped up to about 70V (loaded by my scope and piezo driver) by an auto transformer.

The entire device is really small.

I was thinking that if the frequancy modulation is removed (or new circuitry is built) the auto transformer may be able to produce about 50-60v with enough current to run a small valve device like a single pentode regen.

With the currents involved i'm even sure that a 555 could drive the auto transformer directly, or if not than a bc108 or 2n2222 will ahve no probelm at all.

The device now runs at 4KHz but i think i will play about with a 555 and see what frequancy it works best at.

An interasting idea here. I'm pretty sure the auto transformer has enough current capability to run a DF/DL96 or simular as a regen. Maybe even a ''real'' pentode like a EF91/95. However, if a DL96 is used than it may even produce enough signal to use a xtal earpiece.

Again the entire device can be made REALLY small and run off a 9v battery, using hybrid construction (solid state HT psu, solid state LT psu using LM317, and hollow state detector).

I guess the question is why not just use a transistor regen. I'm afraid i cannot answer that question

.

23rd Oct 2006, 6:35 pm	#1
adibrook Retired Dormant Member Join Date: Dec 2004 Location: Weymouth, Dorset, UK. Posts: 377	compact HT generator idea Today i took a little trip to the local 99p store to buy some of those useless door alarms. They are magnetically activated beepers that go on your door and suppoedly squeal when the door is opened and the magnet moves away. Although most of the time they just squeal all the time even when the magnet is by them. But...i just seem them as a cheap source of piezo drivers. Where else can you get 3 piezo drivers for 99p? Today however i bothered to look at the circuitry instead of just rippign it out. It turns out that its a miniture HV generator to power the piezo driver. Since piezo drivers are high impeadence i guess it makes more sence to run them at high voltage. So i connected a scope to the piezo driver. And...what do you know...it was hitting 70v! It seems that the little IC in the alarm produces a 4KHz pulse which is FM modulated to about 2Hz or so to give the distinctive alarm sound. This is then stepped up to about 70V (loaded by my scope and piezo driver) by an auto transformer. The entire device is really small. I was thinking that if the frequancy modulation is removed (or new circuitry is built) the auto transformer may be able to produce about 50-60v with enough current to run a small valve device like a single pentode regen. With the currents involved i'm even sure that a 555 could drive the auto transformer directly, or if not than a bc108 or 2n2222 will ahve no probelm at all. The device now runs at 4KHz but i think i will play about with a 555 and see what frequancy it works best at. An interasting idea here. I'm pretty sure the auto transformer has enough current capability to run a DF/DL96 or simular as a regen. Maybe even a ''real'' pentode like a EF91/95. However, if a DL96 is used than it may even produce enough signal to use a xtal earpiece. Again the entire device can be made REALLY small and run off a 9v battery, using hybrid construction (solid state HT psu, solid state LT psu using LM317, and hollow state detector). I guess the question is why not just use a transistor regen. I'm afraid i cannot answer that question .