Modified version of the Clone PI-W
The 14th version of the circuit board turned out to be quite successful. Today, the commercial attractiveness of the device has significantly decreased – even an increase in the price of scrap metal does not lead to demand for this device among traders. I hardly see metal hunters in the fields anymore, and over the past couple of years, only a few “Clones” have been brought in for repairs. The forums show almost zero activity in topics related to this metal detector. I believe this allows for sharing materials on the modified device freely. Unfortunately, there is no image of the assembled 14th version board, but a photo of the 12th version, which is quite close to the 14th, was found – Clone_Pi_W_M_12.jpg. The files of the modified circuit, board layout, and assembly drawing are in the attached archive.
Let’s consider the changes to the circuit. The expensive IC keys were replaced with 4066 with level converters on transistors. Field-effect transistors 2N7002 or similar are used as level converter transistors. A regular bipolar transistor is used in the control circuit of the output stage, any n-p-n transistor that is readily available. The use of a bipolar transistor, rather than a field-effect transistor (as in other converters), helped reduce the level of switching noise emitted by this functional node into the common power supply circuits.
As a power transistor, a cheaper type like 4N60 was used instead of IRF740. In the original device circuit, a field-effect transistor was used in the SOUND key. Several “Clones” with a burned-out “sound” output of the microcontroller (pin 16) were repaired several times, requiring the replacement of the “stone” – the most expensive element of the circuit. The burning of this output was due to the breakdown of the field-effect transistor – and it is directly connected to pin 16 of the microcontroller, with the source almost directly connected to 12 volts. In the modified circuit, a bipolar transistor is used, isolated from the microcontroller output by a current-limiting resistor with a resistance of 3.9 to 5.6 kΩ. Any medium-power n-p-n transistor will work (KT645, 2SC945, etc.).
Optimized board layout allowed for a complete elimination of the sample voltage source on TL431, which is also a weak link in the circuit. Now the Uref outputs of the analog part are directly connected to +5 V. This does not affect the operation of the device in the range of smoothly changing supply voltages from 8 to 16 volts. The need for capacitor C15 with 470 μF has also become unnecessary and even harmful – it is sufficient to install no more than 100 μF, and there is no need to increase the capacitance of C1 from 2200 μF to 4700 μF. Other blocking capacitors: C9 – 220 nF, C10 – 100 nF. The modification of the device board allows for lowering the barrier, increasing sensitivity, enabling an increase in its dynamic range. Now the measurement capacitor C6 is not set to 2200 pF (or even 1500 pF), but its capacitance is increased to 3300 pF! The assembly drawing also includes a place for capacitor C* – bandwidth trimming of the input amplifier. Its capacitance is 470-1000 pF; in the extreme versions of the boards, the need for installation has been eliminated. The size of the board has been significantly reduced.
The microcontroller is left in DIP form for the convenience of programming in a separate programmer with the already programmed one installed on the board and for the convenience of board layout. There were many complaints from those who assembled the TL074 op-amp in DIP form due to frequent encounters with low-quality specimens. In SMD form, these chips are much higher in quality, as are the 4066 and 2N7002 field-effect transistors. Therefore, they are applied in SMD packages. The use of resistors – some in SMD form, others in DIP – was determined solely by the convenience of board layout (it is easy to run one, two, or three printed conductors between the pins of a regular resistor, which shortens the conductors themselves). In SMD form, the following are soldered: 4 pieces of 10 kΩ, 2 pieces of 20 Ω, and 1 piece of 3 kΩ. On the DIP resistor, a 2 MΩ resistor is made up of two connected in series at 1 MΩ. There are 5 wire jumpers on the board, one of them is under the microcontroller case.
The first versions of the boards did not start well with new firmware. For this version of the board, it is even recommended to use the firmware “from the fresh” – 1.2.2m (available in the archive). Configuration bits during programming are the same as for previous versions of the metal detector.
In general, assembly is carried out primarily following the assembly drawing (please do not ask questions like “give a list of parts”). The device and its careful assembly are not for absolute beginners.