Coil Operation Frequency and Duty Cycle sensing.

Assuming that the footswitch is continuously pressed down,that leads to the fact
that there's going to be a stable (ideally) voltage dynamic difference between both ends of clipcord.

Simply put ,as long the footswitch is pressed ,
there's always voltage at the clipcord,
thus also at the binding posts of a tattoo machine ,
where the clipcord is attached to .
There's is not a "switching on & off  state",
 of the tattoo coil  machine that can be measured along the clipcord ends.

When the contact screw is in contact with front spring,
el. current flows through the coils, producing a magnetic field around them.
When the coils charge and pull the armature bar downwards,
it's contact with the contact screw open, interrupting current through the coils
and causing the magnetic field to rapidly collapse.

Because the voltage induced in a coil of wire is directly proportional to the rate of change over time of magnetic flux (Faraday's Law: e = NdΦ/dt), this rapid collapse of magnetism around the coil produces a high voltage “spike”.The contacts (contact screw & front spring ),will get excessive arcing at them,  which  greatly reduces their service life.

Either a "surge capacitor " or a " commutating diode " ,
has to be tied parallel to contacts ,to mediate this phenomenon.


In order for the operational frequency and duty cycle to be measured,
a sensing device or mechanism must be " tied " ( connected ) at the very same place ,
where the actual mechanical switching of the coils  is taking place .
At the front spring's  "contact screw " .

There are other ways to sense operational variables ,also.

Like by  measuring the characteristics of the 'kickback' current spikes, 

induced by the collapsing magnetic field of coils , 

when front spring loses contact with the "contact screw ' . 

But while from spike to spike is a full cycle ,making frequency measuring  

quite precise ,measuring the actual duty cycle time duration might prove somewhat  

relative and prone to alterations from various interferences ,

that could cause enough declinations / fluctuations, between measured and actual values.

So this method was avoided,although  being more "convenient" as it does not need an external extra sensor wire,but offers rather reduced  Duty Cycle measurement  precision,also .

Another way is measuring the power line current fluctuctions.
But in that method also,measurements with serious declinations from actual values  ,
may be obtained, mainly due to the fact that charging of coils is not linear,
thus monitoring precisely the current running through power lines ,
might prove quite tricky to translate onto precise Frequency and Duty Cycle figures. 
Moreover a more complex & sensitive sensoring circuitry, of rather higher cost is needed (Hall effect sensoring ).
While still being somewhat  prone to any kind of E/M interference.

Contact screw sensor circuitry :

When the Front Spring contacts the Contact Screw:

Coils are charging.The "HIGH"  duration  depends mostly on the
deflection caused by the rear / back spring.

The higher deflection  of the back spring ,
the longer the front spring will remain in contact with the contact screw.
The longer the coils will charge.
The higher the velocity  with which , the needles will pop in  & out of the skin.
Needle stays off the skin most of the time of a cycle,while popping in and out ,quite 'clean' (with high velocity).

The opposite happens with low back spring  deflection.
Needle will stay longer inside skin ,penetrating it with " dull " punctures of low velocity .
Excessive bleeding might occur,especially with a 'fast working hand '.
Healing times are quite  prolonged.

While coils are charging ,no current runs through the sensor wire .
(Due to sensor's high input impedance and coil's large current draw ).
Sensor 'reads' a "LOW" signal,when actually the machine state is "HIGH" ( coils charging).

When the Front Spring loses  contact with  the Contact Screw:
Magnetic field is collapsing.
A brief (rapid) current spike 'kicks back", onto the power lines.
Some of the spike kickback current  gets" absorbed " by the tattoo machine capacitor.

( Which in turn, will discharge keeping the coils "active" ,
thus holding  the armature bar briefly "stuck" with the front coil's core.
The bigger the capacitor ,the longer the armature bar will stay -briefly- "stuck" at the front coil's core).

Since the footswitch is still pressed down,and there's voltage at the clipcord ends ,
but there's no closed-circuit charging the coils ,at this point a current flows along the sensor line.
Meaning that while the machine state is "LOW" ,the sensor reads "HIGH".


So ,there's an OPTICAL COUPLING LOGIC GATE  INVERTER
  circuit involved ,in order  to invert the sensor's input signal [ Rxi ]* ,
 for  it's output signal [Txo]**  to match the state of the tattoo machine,
while optically (and not electrically ) "coupling " the received signals,
with the transmitted- and inverted-  output signals,
 going into the microcontroller's dedicated input.
Ensuring 100%  noiseless signal  transmission and precise measurements.

Turn- On & Turn-Off delays of the  
OPTICAL COUPLING LOGIC GATE INVERTER Circuitry,
have been  taken into consideration also ,while calculating HIGH/LOW values.
Even if they are falling in the range of just a few millionths of a second!
In order to achieve as higher level of  measurement precision,as possible .

*Rxi: ' Receive signal  (x=1 or 0 - high or low ) input '
**Txo : ' Transmit signal (x=1 or 0  - high or low ) output '