# Radar detector line signal to power a piezo buzzer

#### bigsqueaks

Joined Aug 11, 2016
2
My level of experience in building electronics circuits is novice. I've designed another circuit for my motorcycle which utilized a transistor to change the brightness of some running lights depending whether the brakes are pressed or not.

Now, I'm trying to build a small circuit to connect to the line signal of my radar detector and drive a piezo buzzer. This buzzer requires only 9-12Vdc to produce full volume. I found a circuit to copy here: http://www.zen22142.zen.co.uk/Circuits/Misc/audio_ind.htm .

I used LTspiceXVII to build and simulate the circuit so I know what's going on and that it should work. Below is the circuit drawn in LTSpice. I recorded using my laptop's microphone the startup cycle of the radar which produced a nice wave file. LTSpice sets the input of a wav range to +/- 1V, which I figure is close enough to the measured +/-1.2v line level from the radar detector.

The simulated circuit is slightly different from the linked circuit because I don't have any 10uF ceramic capacitors for C1 and to use a dielectric wouldn't be suitable for the negative voltage it would be subjected to because the line signal from the radar detector is -1.2V to 1.2V approximately. I've read that reverse biasing a dielectric can damage it. I have tried with and without C2 connected in the real circuit.

I understand fairly well what is going. C1 decouples the input so that the inverting input to the op amp oscillates around 6V. The output of the op-amp is amplified to charge and discharge the C3 capacitor, producing a positive voltage at the anode of diode D1 during half the cycle. Then C4 stores some charge and discharges through R6 and therefore is amplified by Q1. R6 also protects Q1 from overcurrent due to the base emitter junction acting like a diode. I will attach my piezo buzzer where R7 is, with the limiting resistor in series. It's possible I could remove it if I determine the resistance of the speaker.

For testing purposes I have a LED in series with R7. I expect the LED to turn on and off. For testing, I'm turning the radar detector off and on because it will output test sounds during its startup.

My problem is that the LED by R7 never turns off. Although, I thought I saw the LED changing in intensity on first test of the circuit but now it remains relatively constant even after power cycling everything. (Did I fry a capacitor?) I'm using a basic circuit to test my transistors to make sure I'm not frying them. It's very easy to fry them... If I accidentally connect +12V directly to the base, I've ruined a transistor. What I've noticed is even when applying ground voltage to the input resistor to the base of a transistor the LED connected to the collector does not turn entirely off. This seems wrong to me if I want to use a transistor as a switch I need the LED to be completely off. The amount of noise when radar detector line level is supposed to be off would have to be enough to cause voltage across D1 to exceed threshold voltage (~.7V).

What I know that I've done is connected C3 the 100uF dielectric capacitor with the wrong polarity when I initially built the circuit. I threw that one away. Positive polarity is towards the op amp.

Tools I have are a digital and analog multimeter and portable oscilloscope that will measure capacitors. I've verified the components in my breadboard's capacitance and resistances are correct. I'm learning to use oscilloscopes with this project.

What troubleshooting steps or does intuition suggest is going wrong? How likely is my op amp still good?