Can't read Russian but the schematic is missing current limiting resistors on the segment lines.
I think he is using the limited current of the computer to limit the LED current.
Do you want the hole thing translated? Here is a start.
The ESR meter is one of the most demanded devices for any radio amateur, and especially for professionals involved in the repair of all kinds of radio equipment. Many variants of such meters have been published in the amateur radio literature. Also, they can be bought relatively inexpensively from China, from where I ordered a device for myself, which burned out safely when a capacitor charged only up to 10V was connected. An analysis of the circuit of this and other similar devices showed their main drawback - the absence or insufficiency of protection against connecting a charged capacitor to the input of the device. This fact led me to the idea of creating a more perfect version of such a device. At the same time, I defined the main required qualities for myself as reliability, autonomy and low power consumption. The main features of the described device: measurement of capacitance and ESR of capacitors without desoldering from the circuit; power supply from a lithium-ion battery from a cell phone with its full service (here it should be noted that the efficiency of the device allows it to be powered from ordinary batteries - alkaline AAA cells can last for six months with an average intensity of use); low current consumption in operating mode and transition to sleep mode with extremely low consumption; quite perfect protection against connecting a charged capacitor; simultaneous indication of both capacitance and the equivalent series resistance of the capacitor or the resistance of the resistor. The device allows you to check capacitors without desoldering, practically in 100% of cases. The voltage applied to the capacitor during measurement does not exceed 0.1 V, which is why all p-n junctions of microcircuits, transistors, etc., including germanium ones, remain closed and do not affect the measurements. The ESR readings of a working capacitor are almost not affected even by ceramic capacitors of much smaller capacitance soldered directly in parallel to the measured part, as well as resistors with a resistance even below 100 Ohm, up to 20 Ohm! Technical characteristics of the described ESR meter (with op-amp AD8032a)
(
Picture here)
The main element of the circuit is a PIC16F873A microcontroller (MC), which is clocked from a crystal oscillator with external elements X1, C9 and C10. The 3.3 V power supply to the MC is supplied from the LDO (Low Drop Out) of the DA2 stabilizer of the XC6206P33 type, which is characterized by extremely low consumption current (1 μA) and minimum voltage drop. The indication is carried out by means of a four-digit red LED indicator, the segment pins of which are connected to the MK port B, and the discharge pins to the RC4-RC7 MK pins. Here, segment-by-segment dynamic indication (DI) is used - only one segment is polled at a time (in a circle, all in 32 cycles). This method of CI made it possible to abandon both bit keys and damping resistors in the segment circuits (I decided to try this method after an interesting discussion with the user vintik in the comments to this article of mine). And this inclusion is quite correct - my measurements with an oscilloscope of current pulses showed: its (current) values (at a supply voltage of 3.3 V) are no more than 12 mA. Which, however, is quite consistent with the graphs of the dependence of the output currents and voltages of the MK ports for 3.3V supply from the datasheet for this MK. In this diagram, the type of indicator is determined automatically, for which, when switched on, the internal pull-up resistors of the MK switched to port B are connected to the segments. The bit pins are connected to the case with RC4-RC7 MK outputs. A signal from one of the segments is fed to the input of the ADC AN3, and by the voltage value, a conclusion is made about the type of indicator, OA or OK. Port RA2, in this case, is switched to input and does not affect the measurement. Since the MC monitors the presence of more than 3.2V at the AN3 input (in the case of an indicator with OA), the process is also not affected by the DA3 TL431A microcircuit connected by the cathode to the battery (even at a current of 0.1 mA, at least 1.8V drops on it, then there is this circuit can provide no more than 2.8V to AN3). Immediately after determining the type of indicator, the RA3 port is transferred to the output and attracted to the case, providing the required current through DA3, and at the AN2 input, either the battery voltage through the R20 resistor or the presence of the charging voltage is periodically measured, minus the 2.5V drop across DA3. Why TL431 and not just a resistive divider? Because in this case, battery current will inevitably leak through this divider in sleep mode, but more on that below ... Actually, the DI is organized in interrupts from the TMR1 timer with an interval of 512 μs. The sampling rate of the indicator is 1000 / (0.512 * 32), approximately equal to 61Hz. The brightness of the indicator, despite the small average current through the segment, is quite sufficient and comfortable. The display system is organized in the device in such a way that two values of four characters, as a rule, capacitance and resistance, are constantly changing on the screen with an interval of 0.5 seconds. When displaying service messages, these values are just two "lines" of the message. The alternating output of these values is performed in the aforementioned interrupts, and the main program simply "puts" the required data in one of the two "cells".
cxem.net
