rwmm415
New Member
A couple of weeks ago I posted here asking for help with repairs to a Dr. Who "talking waste bin". At the time I had thought that the switch sensor -- the mechanism that detects when items are dropped into the bin -- had failed. I figured that the circuit (fig) was good because I could get it to "talk" by shorting the leads to the switch/sensor. I was asking for help in identifying the switch/sensor so I could replace it.
Folks on the list identified it as a piezo transducer that picks up the small vibrations in the brass finger upon which it is mounted. The sensor produces a few hundred millivolts AC when the circular, spring-loaded platform at the bottom of the bin (the surface that the brass finger is mounted on) is disturbed by falling debris, and a volt or more when you thump the brass finger directly with your thumb. From this I concluded that the sensor is probably still good. As I stated above, the circuit I thought was also good. But when you put the two together the system is flaky at best. Sometimes it works pretty well, sometimes not at all. Sometimes it works but with very poor sensitivety (you have to thump the brass finger vigorously). Occasionally it goes off on it's own.
I am asking for help in understanding how the circuit should work. Tracing out the PCB, I sketched the schematic as best I could.
Bear in mind that U1 is one of those mystery ICs that's hidden beneath a glob of epoxy -- so one has to guess at the pin functions by looking at what it's connected to in the external cicuit. In basic terms, transistor Q1 appears to form a common-collector circuit that buffers the output of the sensor and feeds it to pin 1 of the IC. If Q1 is operating as an emitter-follower (I read somewhere that this is a typical use for CC mode circuits) then I would expect to see an AC signal similar to the output of the sensor, lowered by the 0.7V emitter junction drop. But that's not what you see -- somehow the circuit behaves more like a switch. The voltage at the emitter rests at 0.6V when the sensor is quiet and goes to about 1.8V when sensor is struck. When sensor is removed and the circuit is triggered by shorting the connection points, the same voltage pattern is seen on the emitter with quiet and active voltages of 0.5V and 2.5V, resp.
I just don't understand the interface circuitry that connects the piezo to the IC. What I can remember from an intro transistor circuit class in college was mostly about small signal amplifiers running in common-emitter mode. The circuit present in the waste bin is unfamiliar. What I'm hoping is that someone on the forum might recognize this circuit and be able to explain its operation or point me to some existing write-up of a simaliar design.
Also, if you have a hunch as to where the fault lies, please do share that. Since I'm not certain how to fully bench test the piezo sensor, I'm not really 100% sure about it, nor do I know how to obtain or fashion an equivalent to use for a replacement test. The waste bin worked really well when it was new so I'm still hoping to identify the failed component, replace it and get back that original awesomeness.
Many thanks for reading this far and offering any thoughts you might have.
Bob in San Jose
p.s. - all the components are SMDs -- you can see this in the PCB view. I don't know the values of C1 and C2, they have no markings. I realized after completing the schematic that I had reversed the position C1 and C2 from the actual PCB. But since we don't their know values it doesn't make any logical difference.
Folks on the list identified it as a piezo transducer that picks up the small vibrations in the brass finger upon which it is mounted. The sensor produces a few hundred millivolts AC when the circular, spring-loaded platform at the bottom of the bin (the surface that the brass finger is mounted on) is disturbed by falling debris, and a volt or more when you thump the brass finger directly with your thumb. From this I concluded that the sensor is probably still good. As I stated above, the circuit I thought was also good. But when you put the two together the system is flaky at best. Sometimes it works pretty well, sometimes not at all. Sometimes it works but with very poor sensitivety (you have to thump the brass finger vigorously). Occasionally it goes off on it's own.
I am asking for help in understanding how the circuit should work. Tracing out the PCB, I sketched the schematic as best I could.
Bear in mind that U1 is one of those mystery ICs that's hidden beneath a glob of epoxy -- so one has to guess at the pin functions by looking at what it's connected to in the external cicuit. In basic terms, transistor Q1 appears to form a common-collector circuit that buffers the output of the sensor and feeds it to pin 1 of the IC. If Q1 is operating as an emitter-follower (I read somewhere that this is a typical use for CC mode circuits) then I would expect to see an AC signal similar to the output of the sensor, lowered by the 0.7V emitter junction drop. But that's not what you see -- somehow the circuit behaves more like a switch. The voltage at the emitter rests at 0.6V when the sensor is quiet and goes to about 1.8V when sensor is struck. When sensor is removed and the circuit is triggered by shorting the connection points, the same voltage pattern is seen on the emitter with quiet and active voltages of 0.5V and 2.5V, resp.
I just don't understand the interface circuitry that connects the piezo to the IC. What I can remember from an intro transistor circuit class in college was mostly about small signal amplifiers running in common-emitter mode. The circuit present in the waste bin is unfamiliar. What I'm hoping is that someone on the forum might recognize this circuit and be able to explain its operation or point me to some existing write-up of a simaliar design.
Also, if you have a hunch as to where the fault lies, please do share that. Since I'm not certain how to fully bench test the piezo sensor, I'm not really 100% sure about it, nor do I know how to obtain or fashion an equivalent to use for a replacement test. The waste bin worked really well when it was new so I'm still hoping to identify the failed component, replace it and get back that original awesomeness.
Many thanks for reading this far and offering any thoughts you might have.
Bob in San Jose
p.s. - all the components are SMDs -- you can see this in the PCB view. I don't know the values of C1 and C2, they have no markings. I realized after completing the schematic that I had reversed the position C1 and C2 from the actual PCB. But since we don't their know values it doesn't make any logical difference.
