Digital voltage indicator. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Radio amateur designer
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In most cases, analog measurements are best read from a digital display. For this purpose, if necessary, various converters are used (for example, temperature-voltage, phase-voltage), the output signal of which is fed to the ADC and then to a digital indicator. The described device is convenient to use when an inexpensive medium-precision meter is needed, and the use of single-chip ADCs is impossible for some reason.
The diagram of a two-digit indicator is shown in the figure.
The input voltage measurement range is 0...7 V. If the voltage is higher, a divider should be used. The principle of operation of the ADC is based on measuring the charging time of the capacitor to a voltage equal to the measured voltage, and then converting it into digital form. The proportionality of the measured time and voltage is ensured by the stabilization of the charging current.
The operation of the ADC is controlled by a rectangular pulse generator on the elements DD1.3. DD1.4. When a log appears at the output of the generator. 0. transistor VT3 closes, and the inputs PE counters DD2 DD3 operates log. 0. Allowing the counting of pulses from the generator to DD1.1. DD1.2. Capacitor C1 is charged from a current generator on a transistor VT2. When the increasing voltage across the capacitor is equal to the input, the output 9 of the comparator DA1 will appear at a high logic level. Transistor VT1 inverts it. therefore, the operation of the generator on the elements DD1.1 and DD1.2 is blocked. At the same time, at the inputs C DD4. DD5 log is valid. 1, allowing the recording of information from the counters DD2. DD3. The fixed number is displayed on the HG1 LEDs. HG2.
As soon as a log appears at the output of the generator on the elements DD1.3, DD1.4. 1, the transistor VT3 opens and the capacitor C1 is discharged. Comparator DA1 changes its state and blocks the entry into the code converters DD4. DD5. After a short period of time, determined by the R8C4 chain. log. 1 is fed to the PE inputs of the counters DD2, DD3. writing a log to them. 0. After that, the measurement cycle is repeated.
If the voltage at the input of the device is zero, then the output of the comparator DA1 has a high logic level that allows writing to DD4. DD5 and blocking generator on DD1.1. DD1.2. At the same time, counters DD2. DD3 zeros are written, displayed by the indicators.
Structurally, the indicator is made on two boards: on one - LED indicators HG1 are installed. HG2: on the other - all other elements. Mounting on boards can be done by printing or thin wire insulation.
The device uses fixed resistors MLT-0.125, capacitors C2 - C4 can be any ceramic. Trimmer resistor R5 - SP5-2 or other multi-turn; capacitors C1 and C3 are ceramic with a small TKE; K1-73 can also be installed as C17. LED indicators HG1, HG2 can be replaced with ALC324B (with a common anode) by connecting the inputs S of the code converters and the common indicator electrodes to a common wire. Chips DD4. DD5 can be replaced by K176IDZ. Transistors VT1.VT3 - any of the KT315 series.
The adjustment of the assembled device begins with the setting of the charging current of the capacitor C1. To do this, include a microammeter in the gap between the drain of the transistor VT2 and the connection point of the capacitor C1 with the collector VT3 and the selection of the resistor R1 set the current to about 20 μA. After that, a voltage corresponding to the upper limit of the measurement range is applied to the input of the device, and the corresponding indication is set on the indicators with resistor R5. Sometimes, with fuzzy zeroing of the counters (when zero and non-zero readings alternate on the indicators), it is required to select a resistor R8. After adjustment, by changing the input voltage, the operation of the device as a whole is checked.
In the author's version, the described device is used as a voltmeter of a laboratory power supply.
Author: S.Kuleshov, Kurgan
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