NHden
ArboristSite Member
This is a bit long and if you're still interested, read on. I've purchased several low cost tachs in the past year, including this model a month ago. I bought this model based on several good reviews and also chose it as the first to come under my knife. I wanted to examine construction, battery type, and how it operates on the bench (simulated spark source) as the RPMs climb toward the Stihl limit.
The unit was held face down in a milling vise and with a 3/8" end mill, I milled the perimeter of the back, at a depth which I guessed to be wall thickness (20 thousandths or so).
I was rewarded with a case back that fell off in my hands. Inside were multiple treats (for an electro-mech guy). They include:
* very clean assembly, good construction and solder joint quality, very good mounting of the board to the case with numerous screws.
* a board mounted lithium coin cell, either CR2450 or 60 with solder tabs, cleanly attached to the board - can be easily replaced with a small to medium tip soldering iron.
* inside the case, right at the molded wire holes, is a board mounted metal clip (acts as a capacitor to couple the ignition) with trace running to the input circuitry which includes a resistor, several transistors or diodes, other passive components. The purpose of these would be to couple and limit the electric field from the spark so the micro can count or be gated by the signal. The case insulates the metal PCB clip so the spark never enters.
* slapped a piece of self adhesive, polycarbonate sheet over the back hole until I need to continue testing
So, for this model, built in mid year, you get wide open access to the battery by simply milling or cutting off the back of the case.
My theory as to why these counters break up as the coil limits is this. As a frequency counter, realize that 12,000 RPM is only 200Hz, idle around 50Hz. To measure this quickly, you do what lab/bench counter so, you measure pulse period instead of frequency, and you quickly average those readings. In this way, a rapid estimate (with more digits to display) can be had at every ignition pulse. But, if active or passive limiting on the coil caused a funky period to the signal, which I suspect, The frequency displayed will be wildly changing.
Incidentally, the micro has its markings precisely removed. There are data and clock lines but my goal is not to clone this, just examine it.
The unit was held face down in a milling vise and with a 3/8" end mill, I milled the perimeter of the back, at a depth which I guessed to be wall thickness (20 thousandths or so).
I was rewarded with a case back that fell off in my hands. Inside were multiple treats (for an electro-mech guy). They include:
* very clean assembly, good construction and solder joint quality, very good mounting of the board to the case with numerous screws.
* a board mounted lithium coin cell, either CR2450 or 60 with solder tabs, cleanly attached to the board - can be easily replaced with a small to medium tip soldering iron.
* inside the case, right at the molded wire holes, is a board mounted metal clip (acts as a capacitor to couple the ignition) with trace running to the input circuitry which includes a resistor, several transistors or diodes, other passive components. The purpose of these would be to couple and limit the electric field from the spark so the micro can count or be gated by the signal. The case insulates the metal PCB clip so the spark never enters.
* slapped a piece of self adhesive, polycarbonate sheet over the back hole until I need to continue testing
So, for this model, built in mid year, you get wide open access to the battery by simply milling or cutting off the back of the case.
My theory as to why these counters break up as the coil limits is this. As a frequency counter, realize that 12,000 RPM is only 200Hz, idle around 50Hz. To measure this quickly, you do what lab/bench counter so, you measure pulse period instead of frequency, and you quickly average those readings. In this way, a rapid estimate (with more digits to display) can be had at every ignition pulse. But, if active or passive limiting on the coil caused a funky period to the signal, which I suspect, The frequency displayed will be wildly changing.
Incidentally, the micro has its markings precisely removed. There are data and clock lines but my goal is not to clone this, just examine it.
