Need help adding gain pot to RevD build

[yatahaze323]

Just as the title suggests I wanted to add a Gain potentiometer to my RevD build and I'm not exactly sure what resistor to replace and I'm not exactly sure how to connect the pins on the pot to the board.

When looking at the revE build doc the Gain pot is 22k, on the RevD build doc there is a 22k resistor at R11. Is this the resistor that I should replace with a pot?

If so, how do I connect the pins?

Thanks!

[Qiaozhi]

Just as the title suggests I wanted to add a Gain potentiometer to my RevD build and I'm not exactly sure what resistor to replace and I'm not exactly sure how to connect the pins on the pot to the board.

When looking at the revE build doc the Gain pot is 22k, on the RevD build doc there is a 22k resistor at R11. Is this the resistor that I should replace with a pot?

If so, how do I connect the pins?

Thanks!

The so-called "Gain pot" is actually the sample pulse width. Adjusting the width of this pulse can be used as an effective gain control. However, please be aware that the gain versus pulse width is not linear.
On a REV-D board you can replace R11 (22k) with a fixed resistor (4k7) in series with a 25k pot. However, this will provide you with minimal, or at least, no discernible increase in depth, if that's your intention. However, it will allow you to decrease the gain. The REV-D board is already set at maximum gain.

[yatahaze323]

Thanks for clearing that up for me Qiaozhi. Seems like a lot of work for little to "gain" from it.

I'm going to be using the mpp only on the beach is there anything I can do to the board to increase the depth for the beach?

[Qiaozhi]

Thanks for clearing that up for me Qiaozhi. Seems like a lot of work for little to "gain" from it.

I'm going to be using the mpp only on the beach is there anything I can do to the board to increase the depth for the beach?

Have a look at the Build Document for REV-E, in particular step 3, and the Technical Description on page 19.
Reducing the TX pulse rate, increasing the TX pulse width and using a larger coil, can increase the depth for coins by 1 or 2 inches when compared to REV-D.

[yatahaze323]

Ok, I'll check out the revE build doc regarding those. Thanks again for your help!

[yatahaze323]

One more quick question, would there be any point in replacing the 250k threshold pot with a 220k one?

[waltr]

One more quick question, would there be any point in replacing the 250k threshold pot with a 220k one?

None I see since this pot is simply connected to +/- 5V and provides an offset Voltage into the op-amp.

[yatahaze323]

Have a look at the Build Document for REV-E, in particular step 3, and the Technical Description on page 19.
Reducing the TX pulse rate, increasing the TX pulse width and using a larger coil, can increase the depth for coins by 1 or 2 inches when compared to REV-D.

I'm currently comparing Step 3 on REV-E to the REV-D to be able to adjust the pulse width and the pulse rate.

Compairing the two it looks like on my REV-D board I need to replace R7 with a 100k trimmer to adjust the pulse width.

For the pulse rate it shows that I would need to replace R8 with a 1m trimmer with a diode connecting to Pin3 on U4. However the REV-D doesn't have a spot for the diode, do I need to include the diode to be able to reduce the pulse rate?

Thanks again for your help!

[Qiaozhi]

I'm currently comparing Step 3 on REV-E to the REV-D to be able to adjust the pulse width and the pulse rate.

Compairing the two it looks like on my REV-D board I need to replace R7 with a 100k trimmer to adjust the pulse width.

For the pulse rate it shows that I would need to replace R8 with a 1m trimmer with a diode connecting to Pin3 on U4. However the REV-D doesn't have a spot for the diode, do I need to include the diode to be able to reduce the pulse rate?

Thanks again for your help!

The purpose of the second diode is to isolate the pulse rate and pulse width adjustments so that they don't interfere with each other.