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  #1  
Old 01-27-2012, 05:02 PM
drparkwood drparkwood is offline
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Default LM741 Op Amp Voltage Follower Help

Hello,

I am an extreme Novice at this, but I have a feed back issue with my motorcycle cruise control.

I had to tap into the VSS (vehicle speed sensor, pulse generator) which pulsed 0v to 5v with the frequency of pulses dictating the speedometer indicated speed.

When I tapped into this, I beleive the signal is weakenought and there is slight interferance on the original circuit. The recommendation I recieved was to install a voltage follower to buffer and strength the pulse signal.

My question is to build a voltage follower is all that I need the Op Amp or are there any other components.
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Old 01-27-2012, 05:44 PM
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When a 741 opamp is powered by +12V and 0V then its inputs do not work from 0V to +4V and from +8V to +12V. Its output will not go lower than about +2V.

When an MC34071 opamp is powered from +12V then its inputs work all the way down to 0V but do not work from +10V to +12V. Its output goes down to +0.3V.
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Old 01-27-2012, 06:34 PM
jimkeith jimkeith is offline
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I would try a voltage follower--requires only (1) transistor (e.g. 2N3904) and (1) resistor (e.g. 1K)
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Old 01-27-2012, 07:14 PM
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An NPN transistor emitter-follower changes the voltage. Its output is about 0.65V lower than its input.
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Old 01-27-2012, 07:27 PM
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An op-amp is all you need. You can use an LM358, or the quad version LM324. These op-amps work well with single power supplies and the output can go down to 0V (both inputs can sense very close to 0V either). Just tie the output to the negative input and you are set.

You can use the 12-13V coming from the battery to supply the chip. The LM358/LM324 will withstand up to 16V. Use an 100uF 25V electrolytic capacitor and a 100nF 63V polyester capacitor in parallel with the chip. This measure will filter (bypass) any oscillations on the supply.
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Old 01-27-2012, 08:13 PM
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Quote:
Originally Posted by cumesoftware View Post
An op-amp is all you need. You can use an LM358, or the quad version LM324. These op-amps work well with single power supplies and the output can go down to 0V.
No.
Their output does not go down to 0.0V. With a 5.0V supply and a 10k load to ground its output low voltage is typically 5mV but could be as high as 20mV.

A Cmos opamp has an output that can go down to 0.0V when it is not sinking any current.
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Old 01-27-2012, 10:23 PM
BJT_user BJT_user is offline
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I just looked up the specs for the LM358 Low Power Dual Operational Amplifier, as Cumesoftware suggested, and it appears to be just what you need. It is very common and costs little, howerver, be aware that these are very low-power op amps, with their outputs only being able to supply around 20ma. This should not be a problem though, since all you are driving is the input of a pulse detector. You will need a few extra components, as Cumesoftware has stated. A few supply decoupling caps, and additionally, as it states in the datasheet linked above, a reverse voltage protection diode is needed because should the power supply for the op-amp ever become reversed in polarity, an unlimited current surge through the resulting forward diode within the IC could cause fusing of the internal conductors and result in a destroyed unit.
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Old 01-28-2012, 02:53 AM
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I have one comment on this entire thread so far:

www.ti.com/lit/ds/symlink/sn7407.pdf

All you need is +5 for the chip...and pull-up resistor...unless the V_low_output voltage of 0.7v bothers you,...then add a diode and resistor to drop it to approximately zero.

If the circuit that looks at the sensor output in the first place is transistor-transistor logic, 0.7v IS logic low, anyway.
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Last edited by PaulEE; 01-28-2012 at 02:59 AM.
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Old 01-28-2012, 10:17 AM
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Quote:
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If the circuit that looks at the sensor output in the first place is transistor-transistor logic, 0.7v IS logic low, anyway.
Indeed, I didn't remembered that the circuit could be digital. A 0.7V at a TTL input is indeed logic low. The OP could give more details about the circuit. A detailed photo would be nice.
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  #10  
Old 01-28-2012, 01:20 PM
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I was thinking that since the signal is for a tachometer then the output must go down to close to 0V for an accurate idle reading. but now I read that the frequency of the pulses, not their peak voltage determines the RPM reading.

We don't know what the voltage limits are for the pulses and we don't know why there was interference when the extra circuit was added.
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