First of all, thank you all for this forum - I really appreciate the help.
So, I've designed a very simple circuit using two strain gauges in Wheatstone bridges to turn an alarm on and off. Basically, when a load is applied to the bridges, the alarm turns on, and when the load is removed, the alarm turns off. A POT is added at the 741 op-amp to allow crude calibration for different desired weights.
My basic problem is with the op-amp. Since strain gauges produce very small voltage outputs, I obviously need to amplify the signal in order to trigger the attached transistor. Since I want to make this very sensitive, I went with an exponential op-amp setup using a loopback with a diode. However, I can't get anything out of this circuit. When I apply a load to the strain gauges, nothing happens to the output voltage.
My brother in law suggested that I need two op-amps. Since I am using two strain gauges, I need to use one op-amp as an adder and one as an amplifier. I don't see why since the op-amp isn't really adding - tying the strain gauge output together simply combines the input to the op-amp. My thoughts were that I should be tying one of the op-amp inputs to the negative of my power source (a simple wall-wart 9V inverter) instead of to the negative of the strain gauge output. Any ideas?
Lastly, I'm a ME, not an EE, and I never got very far in electronics. Is any type of diode okay? I just bought a pack at Radio Shack - I didn't know if there is really a difference between them.
Sorry the diagram is rough - the program doesn't allow rotation at 45 degrees.
So, I've designed a very simple circuit using two strain gauges in Wheatstone bridges to turn an alarm on and off. Basically, when a load is applied to the bridges, the alarm turns on, and when the load is removed, the alarm turns off. A POT is added at the 741 op-amp to allow crude calibration for different desired weights.
My basic problem is with the op-amp. Since strain gauges produce very small voltage outputs, I obviously need to amplify the signal in order to trigger the attached transistor. Since I want to make this very sensitive, I went with an exponential op-amp setup using a loopback with a diode. However, I can't get anything out of this circuit. When I apply a load to the strain gauges, nothing happens to the output voltage.
My brother in law suggested that I need two op-amps. Since I am using two strain gauges, I need to use one op-amp as an adder and one as an amplifier. I don't see why since the op-amp isn't really adding - tying the strain gauge output together simply combines the input to the op-amp. My thoughts were that I should be tying one of the op-amp inputs to the negative of my power source (a simple wall-wart 9V inverter) instead of to the negative of the strain gauge output. Any ideas?
Lastly, I'm a ME, not an EE, and I never got very far in electronics. Is any type of diode okay? I just bought a pack at Radio Shack - I didn't know if there is really a difference between them.
Sorry the diagram is rough - the program doesn't allow rotation at 45 degrees.
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