Output of Non-Inverting Op-Amp (UA741) Fluctuates and does not go stable

Thread Starter

Wilter.Flow

Joined Apr 7, 2018
8
I have this Diagram, PIR Sensor > Non Inverting Op Amp.

VCC = 12V
Rf = 4k ohms
R2 = 2 ohms
+in = 3V

(The +in is the Output of PIR Sensor going to the Op Amp)

I calculated the Vout to be 9V. The simulation has no problem either. But When I tried it in breadboard, it fluctuates and does not go stable .Capture.JPG
 

Ylli

Joined Nov 13, 2015
1,053
Start out by adding a bypass cap, 0.1 uF is a good value, between the 12 volts and ground right at the opamp. Is that switch ever going to be open? You do not want to have the amplifiers + input floating. Try a 100k to ground and a 100k to + 12 V. Depending on the impedance of your source, that may or may not change the gain a bit.

If that doesn't give you a stable circuit, then you may need to change the opamp type. A 741 is not really designed for single supply and 12 volts (equivalent to +/- 6 volts) may not be enough. Also, it is NOT rail to rail, so don't expect the output to get any lower than about 1 volt or so. I didn't dig up a data sheet.
 

Thread Starter

Wilter.Flow

Joined Apr 7, 2018
8
Sorry I'm just new to this. Those resistors are computed for a 9V output Voltage. That Switch is just for the Portrayal of PIR sensor being On and Off because there is no PIR Sensor to my simulator.

Thanks for your response. I'll try adding ceramic cap between the 12V Source and the Op Amp. I'll be right back :)
 

crutschow

Joined Mar 14, 2008
27,945
A 741 is marginal with a 3V signal. It's input and output do not go much below that value with a single supply voltage.
Try an LM324 op amp instead. It's designed for single supply operation.

Also you need to add a 10kΩ or so resistor from the (+) op amp input to ground so there's a path for the bias current when the switch is open
 
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