Tachometer Voltage Divider Resistance & spurious pulses

Thread Starter

bill2009

Joined Apr 17, 2009
31
I'm pulling a signal from my motorcycle tachometer circuit. it seems to be a 50% duty cycle square wave at battery voltage.

I needed to bring it down to 5v and, not wanting to interfere with the ignition i used 1M & 470K resistors. It works great but I find i get occasional spurious pulses, maybe once in 20 revolutions.

They currently get filtered out by the software but I wondered if they might be a result of using too high a resistance in the interface - there's presumably lots of EMF floating around the coils and maybe the 1M resistor lets it show up as detectable voltage.

I'm re-doing the circuit so I thought I would ask for advice - go down to 100K/47K? lower? put in a small cap on the microprocessor side of the divider?
 

bertus

Joined Apr 5, 2008
22,270
Hello,

Can you post a schematic of what you have upto now?
How did you "clip" the signal at the input of the mcu?

Greetings,
Bertus
 

Thread Starter

bill2009

Joined Apr 17, 2009
31
Hello,

Can you post a schematic of what you have upto now?
How did you "clip" the signal at the input of the mcu?
Yes sure. The signal's not really "clipped", it's just taken down by a factor of 3 which keeps it in the 5v range of the mcu.
 

bertus

Joined Apr 5, 2008
22,270
Hello,

How many volts do you expect at the mcu input?
You could lower the 1M resistor to 10K and replace the other resistor with a zener-diode of the wanted input voltage.

Greetings,
Bertus
 

Thread Starter

bill2009

Joined Apr 17, 2009
31
Hello,

How many volts do you expect at the mcu input?
You could lower the 1M resistor to 10K and replace the other resistor with a zener-diode of the wanted input voltage.

Greetings,
Bertus
i want either 3.5-5v when the input is 11-14 V, or <3 when it's 0.

The 10k resistor would only pull 1ma or so, so that's presumably still ignorable by the ignition circuit and a 4 or 5 v zener would be good protection - right? Is this less likely to suffer interference from the ignition?
 
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