Daughter's favorite talking waste basket has gone mute

[rwmm415]

Hi Folks,

I'm trying to repair a simple circuit in my daughter's Doctor Who talking waste basket. When you drop stuff into the basket it says charming things like "You are an enemy of the Daleks. You will be destroyed!" .

There is an cheap battery-operated circuit with a custom IC and speaker in the base of the basket (fig. 1). It all seems to be in working order as you can short the switch contacts and get it to cycle thru its various recorded phrases and sound effects.

The problem is with the switch (fig. 2) itself. It consists of a brass finger mounted to the bottom of a spring-loaded circular disc that forms the bottom of the waste basket. In theory, when trash is dropped into the basket, the platform is depressed slightly and this momentarily closes the switch contact and triggers the sound effect. Anyway, that's how it used to work.

There is a small rectangular layer of dielectric on the brass finger. This is coated with a thin conductive film. One switch lead is soldered to the film, and the other to the finger. I suspect parts may have broken off so that part of the switch mechanism is missing.

The switch is inoperative -- it reads a constant 2.7K ohms whether the finger is flexed or released -- and it's not clear how it ever worked. For example, there isn't anything under the finger that it would impinge upon when the bottom of the bin is depressed. I'm not even sure if it was a simple contact switch, maybe something more exotic like capacitive or magnetic switch.

My question is two fold: (1) can you guess from these details how this switch once worked, or (2) if you have a similar talking waste basket can you take a look at yours and describe how a functioning version of the switch (and spring-loaded platform) is supposed to operate?

BTW... I'm new to ETO, so if I'm not posting this in the right place or not following some convention please let me know.

Many thanks...

Bob in San Jose

 

[tronitech]

Is it perhaps a strain gauge on the brass strip that gives out a signal when the strip is slightly bent?

 

[alec_t]

... or perhaps a piezo device acting as a vibration sensor?

 

[cowboybob]

Welcome, rwmm415!

Can you take some closeup pix of the switch?

Since a simple, momentary short across the PCB solder points for the switch board's leads activates the unit, the switch itself can't be all that complicated.

 

[rwmm415]

Welcome, rwmm415!

Can you take some closeup pix of the switch?

Since a simple, momentary short across the PCB solder points for the switch board's leads activates the unit, the switch itself can't be all that complicated.

Thanks, all. Here's a close-up of the switch finger where the leads attach (see figure above to understand orientation of close-up). I don't see any sign of a strain gauge foil pattern on the dielectric. Not sure how to test the piezo theory.

**broken link removed**

 

[debe]

Its what Alec-t said, its a piezo sensor. Not hard to test, Use meter set on AC & very low volts & connect to the leads. When you tap the piezo genly with a screw driver you should see about .1 to .2V ac

 

[rwmm415]

Its what Alec-t said, its a piezo sensor. Not hard to test, Use meter set on AC & very low volts & connect to the leads. When you tap the piezo genly with a screw driver you should see about .1 to .2V ac

OK, Debe. You and Alec-t are right. It's a piezo. When I tap the plate upon which the sensor is mounted I get about 0.3-0.4 VAC at about 100 Hz that dies out in about 300 ms. Only problem is that the sensor's output doesn't trigger the circuit even when I thump the disc (the bottom of the waste basket) vigorously. On the other hand, if I short the two leads (labeled BU+ and BU-) on the PCB the circuit works fine. So that still leaves the question -- how did it work originally (when it was new)?

I've included a close-up of the PCB if that provides any additional clues. The two yellow leads from the switch connect at the lower right corner of the board (at BU+ and BU-).

 

[debe]

At a guess check D1 isn't short circuited.

 

[cowboybob]

I also noticed that the PCBs (both) are a tad dirty. You might try cleaning (rubbing) the contact areas (especially in between the traces and the WHOLE piezo bd) with some electronic spray and a Q-tip.

 

[KeepItSimpleStupid]

Or use methanol, or acetone (nail polish remover) on a cotton swap to remove the corrosion.
Acetone destroys most plastics. It generally won't hurt a PCB.

Check the continuity of the wires to the sensor and look for cheap wires (copper and not tin plated). Re-flow or remove and re-strip those wire joints on both ends.

 

[rwmm415]

Thank you, KISS, CowboyBob and Debe. I have cleaned the PCB thoroughly, checked and re-soldered the sensor leads and checked that the diode wasn't short/open. Sadly, the the waste bin still operates intermittently. I have drawn up a schematic for the PCB and posted a lot of additional information in a new thread. I'm going to close this one out now and continue on over there. Thanks again for your insights.