Voltage deviation LED indicator

Encyclopedia of radio electronics and electrical engineering / Protection of equipment from emergency operation of the network, uninterruptible power supplies

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The article describes the design of the indicator, which shows in which direction and how much the value of the controlled parameter has deviated. The device provides a signal output that turns on the actuator. The indicator can be easily reconfigured to work as a signal level meter.

The LED indicator is designed to monitor the deviation of the DC signal from the set value in the positive or negative direction. Assembled on six LEDs, the described version of the device shows three gradations of deviation from "zero" in each direction. The maximum value of the recorded deviation is +0,1 V. The voltage at the inputs should not exceed 3 V, otherwise an input attenuator will be required. The input impedance of the indicator is about 6 kOhm.

The indicator is an integral part of the system for automatically maintaining the specified soil moisture [1] and serves to monitor its deviation from the optimal value.

The scoreboard of the device is a line of six LEDs placed either horizontally or vertically. In the absence of signal deviation from the set value, the three right (or upper) LEDs in the line do not light, the other three are on. With a positive signal deviation, the number of switched on LEDs proportionally increases, with a negative one, it decreases.

The indicator is powered by a stabilized bipolar voltage 2x12 V; consumed current - no more than 40 mA.

The indicator scheme is shown in fig. 1. The controlled input signal is applied to input A. An exemplary voltage is applied to input B. It must be highly stable. It is set equal to the nominal value of the controlled voltage.

Op-amp DA1 is connected according to the scheme with a parallel negative OS. Such inclusion of an op-amp is rarely used due to the presence of a common-mode voltage at the output, but this is quite acceptable for an indicator. The common-mode voltage is compensated when the device is adjusted.

At the output of the op-amp, a bipolar emitter follower on transistors VT1, VT2 is switched on, the output signal of which, through a threshold device (Schmitt trigger), controlled the operation of the irrigation valve of the soil moisture maintenance system. If the device is to be used only as an indicator, the emitter follower can be omitted.

The current amplified signal through the resistor R5 is fed to the circuit of diodes VD1-VD5, which act as stabistors. The voltage drop across each diode is about 0,6 V, which determines the "height" of the indication stage. The voltage distribution in the diode circuit depends on the ratio of the resistance values ​​of the resistors R7 and R8 and the level of the output signal of the op-amp.

In the initial state, at the junction point of the diodes VD3 and VD4, there should be zero voltage relative to the common wire. The bases of transistors VT3-VT5 are under positive voltage, so the transistors VT3-VT5, and hence VT10-VT12, are open. LEDs HL1-HL3 are de-energized, as they are shunted by an open transistor VT12.

Negative voltage is applied to the base of transistors VT7, VT8, and zero voltage is applied to VT6, so they are closed; transistors VT13-VT15 are also closed. The operating current flows through the transistor VT12 and LEDs HL4-HL6 - the LEDs are on.

With an increase in the voltage at input A, the output voltage of the op-amp decreases, the zero voltage point moves to the left along the circuit of diodes VD1-VD5. Pairs of transistors VT5 and VT12, VT4 and VT11, VT3 and VT10 are sequentially closed, stopping shunting the LEDs HL3, HL2 and HL1, respectively. Therefore, the LEDs turn on one by one.

When the voltage at input A decreases, the zero potential point moves to the right along the diode circuit, pairs of transistors VT6 and VT13, VT7 and VT14, VT8 and VT15 open. LEDs HL4, HL5 and HL6 go out one by one.

The LEDs in the indicator are connected in series. The current through them is maintained constant and equal to 10 mA by a current stabilizer on a VT9 transistor. This reduces the load on the power supply and makes it constant. You have to use a pair of transistors to drive each LED, since the amplification of a single transistor is not enough here.

In the journal "Radio" similar LED signal level meters were published [2-5]. Some of their circuit solutions are used in the described device. It differs by the presence of a differential input, a higher and adjustable gain, as well as a series connection of LEDs to a power source, which expands the possibilities of application.

All parts of the indicator, except for the LEDs, are mounted on a single-sided printed circuit board 1 mm thick made of foil fiberglass. The board drawing is shown in fig. 2. The board is made by cutting along a ruler with a cutter, turned on a grinder. The black lines on the board drawing are the areas where the foil has been cut off. Of course, the board can also be made in the traditional way - etching.

The diodes and most of the resistors on the board are installed "upright". The LEDs are mounted on the front panel and connected to the board by a wiring harness.

The transistors used in the indicator can be any low power silicon. For example, instead of KT315B (VT3-VT8), transistors of old types MP113 (with a coefficient h21E> 45) are suitable, and instead of KT502V (VT10-VT15) - MP116 (h21E> 20). To do this, however, will have to slightly increase the size of the circuit board. Transistors VT1, VT2, VT9 must allow a collector voltage of at least 30 V.

Diodes VD1-VD5 - any low-power silicon. The use of the K140UD5 op-amp is not critical - the indicator can also work with another op-amp designed for a supply voltage of 2x15 V. You can change the number of LEDs and, accordingly, the pairs of control transistors.

Setting up the indicator begins with the initial setting of the state of the LEDs. To do this, the inputs A and B are interconnected and an exemplary voltage is applied to them. It is usually formed from the supply voltage using an additional stabilizer and a resistive divider (they are not shown in the diagram). Empirically, on the circuit of diodes VD1-VD5, they find the optimal point, to which they connect the output of the resistor R5, right according to the scheme. The criterion for choosing the optimal point - the LEDs HL4-HL6 are on, and HL1-HL3 are off.

Then, input A is turned off and a stable voltage is applied to it, which can be adjusted within +1 ... 2 V from the reference value. Another similar divider can serve as a source of this voltage, but with a variable resistor in one of the arms. A voltage is set at input A that is exactly equal to the reference one, and the resistor R8 is selected such that, in order to turn on the next LED, it is necessary to increase this voltage by as much as it needs to be reduced in order, on the contrary, to turn off one more of them. The reliability of the indication of "zero" will depend on the thoroughness of this operation.

The required sensitivity of the indicator is set by selecting the resistor R3 in the negative feedback circuit of the op-amp.

If you need to use the indicator in the signal level measurement mode, the inputs A and B are connected to a common wire for adjustment and the resistor R5 is connected to that point of the VD1-VD5 diode circuit, which gives the minimum number of LEDs on. Then the resistor R8 is selected so that the HL6 LED is on the border of the beginning of the glow. Then input B is left connected to the common wire, and a measured positive voltage is applied to input A. With an input signal of opposite polarity, the inputs are reversed. The sensitivity of the indicator is changed by selecting the resistor R3. If the input signal level reaches 3 V, you can do without an operational amplifier and an emitter follower. In this case, a signal source with an internal resistance of not more than 2 kΩ is connected to the circuit of diodes VD1-VD5 through resistor R5.

Literature

1. Egorov Yu., Galitsky V. Electronic control system for irrigation in a greenhouse. - Radio, 1997, No. 11, p. 48, 49.
2. Dimov V. LED signal level meters. - Radio, 1987, No. 10, p. 59, 60.
3. Nechaev I. LED signal level meter. - Radio, 1988, No. 12, p. 52.
4. Potigin I. LED setting indicator. - Radio, 1987, No. 12, p. 39.
5. Yanko B., Potapova L. Combined indicator of the output power of the AF amplifier. - Radio, 1987, No. 8, p. 32, 33.

Author: Yu.Egorov, Moscow

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