undercurrent
New Member
Hi everybody, what I am trying to accomplish is a compact piezo touch switch for LED lamps, 12 volts in, 12 volts out, switching occurs through a touch sensitive piezo buzzer that controls a BC547, which in turn acts as a non-mechanical switch for a typical NE555 bistable circuit.
The circuit I am basing all this of can be found on Google Images: 555flipflop.gif. I've just replaced the mechanical switch basically. The piezo is connected from ground up to a diode to eliminate negative voltages, and a zener to cut out extreme voltage peaks, then going straight into BC547's base.
Obviously I try to keep the component count down, it's supposed to fit into a quite flat cylindrical button 30mm in diameter!
So the problem: I'd like the switch to act on a light tap or touch, and need to bias the piezo element, which will feed out peaks of 0.3-0.5 Volts when unbiased. Because BC547 is expecting a Vbe of at least 0.7 Volts. But the biasing doesn't work. I am pretty new to electronics, I made a simple voltage divider (100kOhm and 10kOhm giving 0.44 Volts) and tried placing it both in parallel and in serial with the Piezo. Tinkered around in all sorts of ways. But that voltage just doesn't seem to add up with the output of the piezo. That bias seems to basically smother out any output from the piezo.
What component is the piezo closest to anyway in it's neutral form, can it be considered a capacitor maybe?
So please advice: how on earth does one bias a piezo buzzer? The problem seems to be really at the piezo bias level, I have checked the signals at different points with my compact Helleman oscilloscope.
By the way, the circuit works just fine without biasing, it's just that the tapping necessary is a bit towards the violent way, and I don't want the button to break too soon!
The circuit I am basing all this of can be found on Google Images: 555flipflop.gif. I've just replaced the mechanical switch basically. The piezo is connected from ground up to a diode to eliminate negative voltages, and a zener to cut out extreme voltage peaks, then going straight into BC547's base.
Obviously I try to keep the component count down, it's supposed to fit into a quite flat cylindrical button 30mm in diameter!
So the problem: I'd like the switch to act on a light tap or touch, and need to bias the piezo element, which will feed out peaks of 0.3-0.5 Volts when unbiased. Because BC547 is expecting a Vbe of at least 0.7 Volts. But the biasing doesn't work. I am pretty new to electronics, I made a simple voltage divider (100kOhm and 10kOhm giving 0.44 Volts) and tried placing it both in parallel and in serial with the Piezo. Tinkered around in all sorts of ways. But that voltage just doesn't seem to add up with the output of the piezo. That bias seems to basically smother out any output from the piezo.
What component is the piezo closest to anyway in it's neutral form, can it be considered a capacitor maybe?
So please advice: how on earth does one bias a piezo buzzer? The problem seems to be really at the piezo bias level, I have checked the signals at different points with my compact Helleman oscilloscope.
By the way, the circuit works just fine without biasing, it's just that the tapping necessary is a bit towards the violent way, and I don't want the button to break too soon!
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