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I am helping my friend set up a 12v actuator on his harvester. I am going to set a limit switch to turn on a indicator light when it reaches end of travel. What would be the simplest way to set up a piezo beeper to chirp once every time the light goes on. Maybe there is a beeper for sale that is designed for that already.
Lamar

You could build one using a 555 timer chip or connect to a back-up beeper such as this one.
http://www.amazon.com/Industrial-Heavy- ... 429&sr=1-8

Steve G

Hi Lamar,

The 555 timer used as a monostable will work. Here's a link that will help. http://555-timer-circuits.uk/operation/one-shot.html

An old school trick is to use a SPDT switch and a BIG capacitor. Power goes to the NC contact. The capacitor goes to the COM contact and the load (beeper) goes to the NO contact. At idle, the cap is being charged. when the contacts switch, the cap's charge is applied to the load, which becomes powered for a time based on the size of the capacitor. If you used a DPDT limit switch, or added a relay, like a 12VDC DIP relay, one contact set would be for the beeper and one for the light.

I have used this idea, with proper parts values, in a coffee pot to signal the end of the brew cycle.

Len

A fragile piezoelectric and 555 circuitry can be avoided for harsh environments with a plain buzzer instead, which is nothing else but an automotive relay with its 12V coil in series with its N.C. contact. And they are sealed, waterproof, probably in your parts bin, and quick to implement.

(+)-------------relay coil-------------common-----------normally closed------------(-)

If you insist on piezo; for $2 ----> http://www.allelectronics.com/make-a-st ... zer/1.html
Or, salvage the chime from a defunct automobile.

Miguel

From the OP...

chirp once every time the light goes on

The buzzer would sound as long as the switch is closed....
Len

Correct, Len, I misread and goofed there.

(+)-----------capacitor------------relay coil-------------common-----------normally closed------------(-)

In parallel to the light.

Externet...
If I understand your description, the relay will energize with the capacitor's charging current and once the capacitor charges the current drops and the relay de-energizes, hence the single beep.

But I do see a problem! Once the capacitor charges it will stay charged as long as power is applied. Unless the off time is long enough for the capacitor to self discharge, there may not be enough charge current the next time for the relay to operate.

The SPDT circuit dumps the charged cap to the load, and when the contacts restore, it recharges. The duration of the operation depends on the length of time the switch is closed and the size of the capacitor.

Actually, I better favor a monostable using a xx555 since there are several benefits to consider. The 555 is cheap, and with the parts around it, only a few dollars are needed plus the beeper; it can be operated directly on 12VDC; it can be piggy-backed to the beeper and encapsulated to make it less likely to be damaged; it will send a pulse of the same duration regardless of trigger duration; the recycle time is nil. If more output for a larger sounding device is needed then a simple relay, or transistor driver could be added.

Thanks for the help guys.

Lenp wrote: Here's a link that will help. http://555-timer-circuits.uk/operation/one-shot.html

That looks like a handy website, would something like that be available as a download so it would be usable for the times you don't have web access?

Lamar

Hi Lamar
There are several Excel spreadsheets that are online.
Right now I am away from computer and this silly Iphone won't run anything like excel.
When I get a chance. I'look at some of them and post back.
Any good tech reference guide for the 555 should show example calculations that could be the basis of a custom spreadsheet.

Here is a link to several downloadable 555 calculator!
http://www.electroschematics.com/6482/5 ... -software/