I have several tachometers for V8 Chevy cars. They all read too high, so I have modified them with a pot in place of the original fixed thin-film resistor.
Now I need a signal source to use as an input. I assume a 12 volt square wave would work. I have assembled an astable multivibrator using a CD4093. I have the output of the oscillator driving a 2N3904, but the tach reads nothing.
The circuit is pretty basic. The oscillator works and I adjust the frequency with a pot to get it set accurately. The 4093 drives the 2N3904 through a 4.7K resistor to the base of the 3904. The 3904's emitter is at ground. I connected the collector to the tach input, but I get nothing. The tach has 12 volts and ground also connected.
I'm wondering if I should have a resistor (possibly an inductor) between the 12 volts and the tach input???
Any help is greatly appreciated.
Jack
NEVER MIND. I added the resistor and now it works!
Now I need a signal source to use as an input. I assume a 12 volt square wave would work. I have assembled an astable multivibrator using a CD4093. I have the output of the oscillator driving a 2N3904, but the tach reads nothing.
The circuit is pretty basic. The oscillator works and I adjust the frequency with a pot to get it set accurately. The 4093 drives the 2N3904 through a 4.7K resistor to the base of the 3904. The 3904's emitter is at ground. I connected the collector to the tach input, but I get nothing. The tach has 12 volts and ground also connected.
I'm wondering if I should have a resistor (possibly an inductor) between the 12 volts and the tach input???
Any help is greatly appreciated.
Jack
NEVER MIND. I added the resistor and now it works!
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