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Here is the latest incarnation of the popular Minipulse Plus design. Although this revision is referred to as MPP Beach, there is nothing to prevent the various settings being adjusted to detect lower conductivity items such as gold nuggets.
Please find the REV-E Build Document below.
As before, I cannot guarantee the document is totally error-free, so please provide feedback if you find something wrong. As you will be aware, it's always difficult to check your own work.
The board size is the same as for REV-D, but with the following modifications:
1. The TX oscillator can now be adjusted by on-board trimmers, with the pulse width and pulse
period being independently adjustable. The range is such that flat-topping the coil current is
possible for most coils.
2. The positions of some components have been adjusted for easier assembly.
3. The sample pulse width is now available as an external 22k pot. This effectively provides
some adjustable gain.
4. TL061 and TL062 opamps have been changed to TL071 and TL072. Silverdog kits were
already being supplied with these parts.
5. Audio threshold pot changed from 250k to 220k.
6. MOSFET changed from IRF740 to IRF840 to increase flyback voltage.
7. 100nF capacitor in audio circuit increased to 470nF.
8. Moved TX oscillator testpoint from input of sample pulse generator to output of TX
oscillator.
9. Changed silkscreen on connectors to show correct orientation. (Vertical bar on inside.)
Note that the current document reference is 20190723.
If you downloaded this document before the 23rd July 2019, then please replace it with the one below. The document reference number can be found at the bottom of each page.
Updates:
Please find the REV-E Build Document below.
As before, I cannot guarantee the document is totally error-free, so please provide feedback if you find something wrong. As you will be aware, it's always difficult to check your own work.
The board size is the same as for REV-D, but with the following modifications:
1. The TX oscillator can now be adjusted by on-board trimmers, with the pulse width and pulse
period being independently adjustable. The range is such that flat-topping the coil current is
possible for most coils.
2. The positions of some components have been adjusted for easier assembly.
3. The sample pulse width is now available as an external 22k pot. This effectively provides
some adjustable gain.
4. TL061 and TL062 opamps have been changed to TL071 and TL072. Silverdog kits were
already being supplied with these parts.
5. Audio threshold pot changed from 250k to 220k.
6. MOSFET changed from IRF740 to IRF840 to increase flyback voltage.
7. 100nF capacitor in audio circuit increased to 470nF.
8. Moved TX oscillator testpoint from input of sample pulse generator to output of TX
oscillator.
9. Changed silkscreen on connectors to show correct orientation. (Vertical bar on inside.)
Note that the current document reference is 20190723.
If you downloaded this document before the 23rd July 2019, then please replace it with the one below. The document reference number can be found at the bottom of each page.
Updates:
- MartJ1963 has successfully completed an MPP Beach, but noticed that C20 was listed as 100nF in the parts list instead of 470nF. The Build Document has been updated, and the new version can be found below.
- 15 August 2018 - Added schematic to attachment list.
- 23 July 2019 - Gain pot was 10k in the Introduction. This has been changed to 22k.
- 13 Jan 2022 - Added mono coil connection diagram (MPP_TX.jpg) to attachment list (drawn by Carl).