Existing PI changes

Audio section

Increase volume. Change R20 to 51 ohms and R18 to 10Kohms.

SAT

R25 from 220K to 270K to increase time

Dampening Resistor

470ohm dampening resistor added across coil. This may vary based on specific coil parameters.

Op Amp

FET input TL072 is used in place of the LM358 at U5

Inputs from Cyril

Dec 14, 2021

James,

Ok, here are some of my thoughts. I see this first cut as a proof-of-concept design. If we can get the functionality control out of the firmware in the Arduino, I will do some redesign changes to the actual PI circuit to take advantages of the functionality control.

Functional control:

1. Change the Target Sample Width, 15us, 30us and 45us

2. Change the audio tone to allow two frequencies, 250 Hz and 500 Hz.

3. Bar graph signal strength, ( you have already demonstrated that function.

Now, if we can achieve the above functionality, I will redesign the PI circuit to take advantage of the new adjustability. this will require redistributing the front to back gain to allow target samples to be take in at around 10us. Thus, when using the 15us sample width the detector will have a lot better response to small gold targets. When Sampling at 45us it will favor the higher conductor silver and copper targets. The two audio tones will make it nice for picking the most comfortable tone for the user and the Signal strength graph will help in Pin pointing targets.

Dec 20, 2021

James,

I decided to address the fairly low audio volume. George Overton uses the mpsa13 as a current source with a 150 ohm resistor in the emitter to drive the headphones  or speaker. This approach is adequate for headphones but lacks volume for a speaker. I changed the board layout and made the mpsa13 into a common emitter amplifier by moveing the 150 ohms resistor to the collector and grounding the emitter. The added gain will improve the signal volume of the deeper weaker targets as well as normal targets. A 1/4 watt 150 ohm resistor will work in this application. The value can be adjusted to the users need.

I am increasing the board and schematic Revision to Rev. 0.4.

Jan 5, 2022

James,

I thought I would recap the functionality items that we talked about for the PI.

1. Coil Pulse width steps, 20, 40, 60, and 80us

1. Target Sample and EFE sample width steps, 20, 30, and 40us

1. Battery voltage read out.

1. Signal strength bar graph.

I think you mentioned that would need a voltage divider from the 12v down to about 5v for the battery voltage read out. There is plenty of room on the board for that. I think you also said that it would need to be connected up to one of the analog pins on the Arduino?  When you pick one let me know and I will look at how to make it happen.

Cyril