Piezo Voltage Acticated Switch

**zotzman** · 27th November 2009 11:06 PM

I have a device that runs on 6Vdc power and transmits a signal when on. Its a beacon for oceanography. This device is normally off until it comes to the surface of the ocean where it must turn on. Other pressure switches have piston activated switches and other ideas that may work 70% of the time, but I think I have come up with something that will do a slightly better job.

The device will sit at the bottom of the ocean and need to be rated at 12000psi. I have though of using a potting compound to contain a piezoelectric crystal. When the device is signaled to release from its anchor it will begin its journey to the surface. As this happens it will experience a negative pressure change and a voltage will be produced until the pressure equalizes. I want to use this voltage to switch on the device so it uses its 4xC cell 1.5v batteries. I can come up with something that uses minimal current because the device will sit in the ocean for up to 2 years before it needs to be turned on.

I am on my last attempt at an alternative before I am forced to redesign the pressure switch and make it purely mechanical and I don't want to do that...I want to come up with something better...

How can I turn on a switch with a small and short voltage that will in turn turn on the device?

**Sceadwian** · 27th November 2009 11:16 PM

Even if it comes up fast the pressure change isn't going to cause that piezo crystal to supply very much current. Personally I think you should redesign the pressure switch. A simple cylinder of sealed ambient pressure air would work just fine, the cylinder would compress when it goes down more than a few atmospheres actuating the switch, and wouldn't extend until it came back up to the surface. I don't understand how that would be prone to failure.

**zotzman** · 28th November 2009 12:09 AM

The Orings often leak and crud can get on the piston and cause it to become immoble. I have seen a cct using a 2N2222 NPN transistor and a sound activated peizo here http://www.hiviz.com/tools/triggers/SNDTRIG.GIF

Maybe I can use this to switch it on? I just need to switch something on so it stays on.

**Externet** · 28th November 2009 01:15 AM

Is the activation of the beacon mandatory needed at the moment it reaches surface or can it be delayed a few hours ?

**zotzman** · 28th November 2009 01:51 AM

Yes, the beacon must turn on when surfaced.

**zotzman** · 28th November 2009 02:00 AM

Weird, I thought I posted this...I found a sound activated piezo cct using a 2n2222 found here.

http://www.hiviz.com/tools/triggers/SNDTRIG.GIF

Make your own

Could I amplify the signal and get a switch activated using an opamp comparitor?

I can understand the concepts of electronics but I still have yet to use it to solve problems. I can come close but this is why I have come here, to the gurus.

I can supply power to it if the amphours are low ie under 20,000.

**zotzman** · 28th November 2009 02:01 AM

I have found a cct that uses a 2n2222 with a peizo sensor. Could it be modified and have a voltage switch on it?

You can find it here:

http://www.hiviz.com/tools/triggers/makeown.htm#st

Thanks.

**Externet** · 28th November 2009 04:30 AM

That schematic seems to be for a strobe light, triggered by a piezopulse.
Is that the beacon meant for your device or is it a radio beacon ?
The quiescent current in that circuit may not leave much life on the cells when the moment arrives. I do not like the brutal flexion a piezodisc would suffer on such pressure differential. I would not be clicky fast either

Honestly am unsure to rely in 2 year old cells to make the beacon work.

Well, as it has to turn-on instantly when surfacing and cannot wait a few hours, my suggestion won't click. I would put small solar cells instead. If surfaces at night, Helius will show up soon. If during the day, you are done. Turns on when sees light.

The circuitry could be made with reverse bias a turn-on mosfet, in series with the solar cell(s) to force forward bias when illuminated. Or something like that; to be polished -latch-on.
The cells encapsulated behind a lens are impervious to pressures, no moving parts. At that depth it is black-black.
Well, good luck, come back with results.

Miguel

**zotzman** · 28th November 2009 04:35 AM

There is something similar to this being used in another industry but I cannot get plans for it. The solar has been explored and risks getting dirty or turning on at low depths. The mA-hrs of the batteries is quite high at about 33000, so I'm sure something could work. This is a puzzle that needs to be solved!

**Externet** · 28th November 2009 04:55 AM

Unavailable plans is not a deterrent. Can be designed to your demands.

**Sceadwian** · 28th November 2009 04:57 AM

I still don't understand why the mechanical pressure switch is failing so frequently. It should be very very simply and reliable.

**zotzman** · 28th November 2009 09:12 AM

Because it has orings and they can get grime in them thus restricting motion and/or if there is anything on the oring that is missed during assembly then they will fail. Orings can also have defects in manufacturing. I wouldn't be going to the extent of asking for help if I didn't think it was worth it.

If you can help me then please help me design an amplifying switch with a voltage switch or any other ideas.

These are failing for whatever reason and a $10,000 piece of equipment is turning to crap. Some of these are failing upon second deployment...after being serviced and it is out of the scope of the warranty because it is now 2 years old.

Please help if you can.

**mneary** · 28th November 2009 09:26 AM

If you told us what the voltage and current are generated by your piezo sensor, we would have provided a circuit already. A data sheet would be best of all!

Since no one here apparently has seen such a sensor, creating a design to match it is impossible.

**Sceadwian** · 28th November 2009 09:38 AM

zotz, what triggers the device to return to the surface?

**zotzman** · 28th November 2009 06:30 PM

The device is triggered to the surface by sending a signal to something called an acoustic release. This acoustic release is attached to the anchor at the bottom.

I have 6Vdc and the piezo can supply a voltage difference of 0.7Vdc.

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