// delay 0.5 sec

1. Connect the potentiometer according to the diagram in Fig. 4.1.
2. We connect the leads of the LED scale with the anode contacts through limiting resistors with a nominal value of 220 Ohms to the Arduino D3-D12 pins, and the cathode contacts to the ground (see Fig. 4.3).
3. Load the sketch from Listing 4.2 onto the Arduino board.
4. Turn the potentiometer knob and observe on the LED scale the level of the potentiometer value from the maximum value.

LED scales are often used to monitor voltage.
Let's consider several ways to construct such schemes.
Passive scales are powered by a signal source and have the simplest circuit.

This could be a car voltmeter. Then VD8 should be selected for 12 volts, since it sets the illumination voltage of the first LED on the scale. The following LEDs VD2 - VD4 are connected via diode junctions VD5-VD7. The drop across each diode averages 0.7 volts. As the voltage increases, the LEDs will turn on one by one.
If you put two or three diodes in each arm, the voltage scale will stretch the corresponding number of times.

According to this scheme, a battery indicator from 3V to 24V is built

Another way to build a line of diodes.

In this circuit, the LEDs light up in pairs, the switching step is 2.5 volts (depending on the type of LED).
All of the above circuits have one drawback - very smooth illumination of the LEDs as the voltage increases. For sharper switching, transistors are added to such circuits in each arm.

Now let's look at the active scales.
There are specialized microcircuits for this purpose, but we will consider more affordable elements that most people have on hand. Below is a diagram of logical repeaters. Logic chips 74ls244, 74ls245 for 8 channels are suitable here. Don’t forget to supply +5 volt power to the microcircuit itself (not indicated on the diagram).

Response threshold of the first element DD1
equal to the logical level for a given series of chips.

If we use inverters of the type K155LN1, K155LN2, 7405, 7406 in such a circuit. The connection will be as follows:

The advantage is that in such a circuit the output works with an open collector, this allows the use of ULN2003 and the like in the assembly circuit.
And lastly, this is the implementation of a running point on logical elements 4i-not.

The logic works in such a way that each element, when turned on, prohibits the operation of all elements of the lowest number. K155LA6 microcircuits are used in this circuit. The last two elements DD3 and DD4, as can be seen from the diagram, can have two inputs, for example: K155LA3, K155LA8.
For battery devices, it is advisable to use low-power analogues from the 176 and 561 series of microcircuits.

Due to such properties as: low power consumption, small dimensions and simplicity of the auxiliary circuits necessary for operation, LEDs (meaning LEDs in the visible wavelength range) have become very widespread in electronic equipment for a wide variety of purposes. They are used primarily as universal operating mode indication devices or emergency indication devices. Less common (usually only in amateur radio practice) are LED lighting effect machines and LED information panels (scoreboards).

For the normal functioning of any LED, it is enough to ensure that a current flowing through it in the forward direction does not exceed the maximum permissible for the device used. If this current is not too low, the LED will light. To control the state of the LED, it is necessary to provide regulation (switching) in the current flow circuit. This can be done using standard serial or parallel switching circuits (transistors, diodes, etc.). Examples of such schemes are shown in Fig. 3.7-1, 3.7-2.

Rice. 3.7-1. Ways to control the state of an LED using transistor switches

Rice. 3.7-2. Methods for controlling the state of an LED from TTL digital chips

An example of the use of LEDs in signaling circuits is the following two simple circuits of mains voltage indicators (Fig. 3.7-3, 3.7-4).

Scheme in Fig. 3.7-3 is intended to indicate the presence of alternating voltage in a household network. Previously, such devices usually used small-sized neon bulbs. But LEDs in this regard are much more practical and technologically advanced. In this circuit, current passes through the LED only during one half-wave of the input AC voltage (during the second half-wave, the LED is shunted by a zener diode operating in the forward direction). This turns out to be sufficient for the human eye to normally perceive light from the LED as continuous radiation. The stabilization voltage of the zener diode is selected to be slightly greater than the forward voltage drop across the LED used. The capacitance of the capacitor \(C1\) depends on the required forward current through the LED.

Rice. 3.7-3. Mains voltage indicator

Three LEDs contain a device that informs about deviations of the mains voltage from the nominal value (Fig. 3.7-4). Here, too, the LEDs glow only during one half-cycle of the input voltage. Switching of LEDs is carried out through dinistors connected in series with them. The LED \(HL1\) is always on when the mains voltage is present, two threshold devices on dinistors and voltage dividers on resistors ensure that the other two LEDs turn on only when the input voltage reaches the set operating threshold. If they are adjusted so that at normal voltage in the network the LEDs \(HL1\), \(HL2\) are lit, then at increased voltage the LED \(HL3\) will also light up, and when the voltage in the network decreases the LED \( HL2\). The input voltage limiter at \(VD1\), \(VD2\) prevents device failure when the normal voltage in the network is significantly exceeded.

Rice. 3.7-4. Mains voltage level indicator

Scheme in Fig. 3.7-5 is designed to signal a blown fuse. If fuse \(FU1\) is intact, the voltage drop across it is very small and the LED does not light up. When the fuse blows, the supply voltage is applied through a small load resistance to the indicator circuit, and the LED lights up. Resistor \(R1\) is selected from the condition that the required current will flow through the LED. Not all types of loads may be suitable for this scheme.

Rice. 3.7-5. LED fuse indicator

The voltage stabilizer overload indication device is shown in Fig. 3.7‑6. In normal mode of operation of the stabilizer, the voltage at the base of the transistor \(VT1\) is stabilized by the zener diode \(VD1\) and is approximately 1 V more than at the emitter, so the transistor is closed and the signal LED \(HL1\) is on. When the stabilizer is overloaded, the output voltage decreases, the zener diode exits the stabilization mode and the voltage at the base \(VT1\) decreases. Therefore, the transistor opens. Since the forward voltage on the turned-on LED \(HL1\) is greater than on \(HL2\) and the transistor, at the moment the transistor opens, the LED \(HL1\) goes out, and \(HL2\) turns on. The forward voltage on the green LED \(HL1\) is approximately 0.5 V greater than on the red LED \(HL2\), so the maximum collector-emitter saturation voltage of the transistor \(VT1\) should be less than 0.5 V. Resistor R1 limits the current through the LEDs, and resistor \(R2\) determines the current through the zener diode \(VD1\).

Rice. 3.7-6. Stabilizer status indicator

The circuit of a simple probe that allows you to determine the nature (DC or AC) and polarity of voltage in the range of 3...30 V for DC and 2.1...21 V for the effective value of AC voltage is shown in Fig. 3.7-7. The probe is based on a current stabilizer based on two field-effect transistors, loaded onto back-to-back LEDs. If a positive potential is applied to terminal \(XS1\), and negative potential is applied to terminal \(XS2\), then the HL2 LED lights up, if vice versa, the \(HL1\) LED lights up. When the input voltage is AC, both LEDs light up. If none of the LEDs are lit, this means that the input voltage is less than 2 V. The current consumed by the device does not exceed 6 mA.

Rice. 3.7-7. A simple probe-indicator of the nature and polarity of voltage

In Fig. 3.7-8 shows a diagram of another simple probe with LED indication. It is used to check the logic level in digital circuits built on TTL chips. In the initial state, when nothing is connected to the \(XS1\) terminal, the \(HL1\) LED glows faintly. Its mode is set by setting the appropriate bias voltage at the base of the transistor \(VT1\). If a low level voltage is applied to the input, the transistor will close and the LED will turn off. If there is a high voltage level at the input, the transistor opens, the brightness of the LED becomes maximum (the current is limited by the resistor \(R3\)). When checking pulse signals, the brightness of HL1 increases if a high-level voltage predominates in the signal sequence, and decreases if a low-level voltage predominates. The probe can be powered either from the power supply of the device under test or from a separate power source.

Rice. 3.7-8. TTL logic level indicator probe

A more advanced probe (Fig. 3.7-9) contains two LEDs and allows you not only to evaluate logical levels, but also to check the presence of pulses, evaluate their duty cycle and determine the intermediate state between high and low voltage levels. The probe consists of an amplifier on a transistor \(VT1\), which increases its input resistance, and two switches on transistors \(VT2\), \(VT3\). The first key controls the LED \(HL1\), which has a green glow, the second - the LED \(HL2\), which has a red glow. At an input voltage of 0.4...2.4 V (intermediate state), the transistor \(VT2\) is open, the LED \(HL1\) is turned off. At the same time, the transistor \(VT3\) is also closed, since the voltage drop across the resistor \(R3\) is not enough to fully open the diode \(VD1\) and create the required bias at the base of the transistor. Therefore, \(HL2\) does not light up either. When the input voltage becomes less than 0.4 V, the transistor \(VT2\) closes, the LED \(HL1\) lights up, indicating the presence of a logical zero. When the input voltage is more than 2.4 V, the transistor \(VT3\) opens, the LED \(HL2\) turns on, indicating the presence of a logical one. If a pulse voltage is applied to the probe input, the duty cycle of the pulses can be estimated by the brightness of a particular LED.

Rice. 3.7-9. An improved version of the TTL logic level indicator probe

Another version of the probe is shown in Fig. 3.7-10. If terminal \(XS1\) is not connected anywhere, all transistors are closed, LEDs \(HL1\) and \(HL2\) do not work. The emitter of the transistor \(VT2\) from the divider \(R2-R4\) receives a voltage of about 1.8 V, the base \(VT1\) - about 1.2 V. If a voltage above 2.5 V is applied to the input of the probe , the base-emitter bias voltage of the transistor \(VT2\) exceeds 0.7 V, it will open and open the transistor \(VT3\) with its collector current. The LED \(HL1\) will turn on, indicating the state of logical one. The collector current \(VT2\), approximately equal to its emitter current, is limited by resistors \(R3\) and \(R4\). When the input voltage exceeds 4.6 V (which is possible when checking the outputs of open-collector circuits), the transistor \(VT2\) enters saturation mode, and if the base current \(VT2\) is not limited by the resistor \(R1\), the transistor \(VT3\) will close and the LED \(HL1\) will turn off. When the input voltage decreases below 0.5 V, the transistor \(VT1\) opens, its collector current opens the transistor \(VT4\), turns on \(HL2\), indicating the state of logical zero. Using resistor \(R6\) the brightness of the LEDs is adjusted. By selecting resistors \(R2\) and \(R4\), you can set the necessary thresholds for turning on the LEDs.

Rice. 3.7-10. Logical level indicator probe using four transistors

To indicate fine tuning, radio receivers often use simple devices containing one, and sometimes several, LEDs of different colors.

A diagram of an economical LED tuning indicator for a battery-powered receiver is shown in Fig. 3.7-11. The current consumption of the device does not exceed 0.6 mA in the absence of a signal, and with fine tuning it is 1 mA. High efficiency is achieved by powering the LED with pulsed voltage (i.e., the LED does not glow continuously, but blinks frequently, but due to the inertia of vision, such flickering is not noticeable to the eye). The pulse generator is made on a unijunction transistor \(VT3\). The generator produces pulses with a duration of about 20 ms, followed by a frequency of 15 Hz. These pulses control the operation of the switch on the transistor \(DA1.2\) (one of the transistors of the microassembly \(DA1\)). However, in the absence of a signal, the LED does not turn on, since in this case the resistance of the emitter-collector section of the transistor \(VT2\) is high. With fine tuning, the transistor \(VT1\), and then \(DA1.1\) and \(VT2\) will open so much that at the moments when the transistor \(DA1.2\) is open, the LED will light up \( HL1\). To reduce current consumption, the emitter circuit of the transistor \(DA1.1\) is connected to the collector of the transistor \(DA1.2\), due to which the last two stages (\(DA1.2\), \(VT2\)) also operate in key mode. If necessary, by selecting a resistor \(R4\) you can achieve a weak initial glow of the LED \(HL1\). In this case, it also serves as an indicator for turning on the receiver.

Rice. 3.7-11. Economical LED setting indicator

Cost-effective LED indicators may be needed not only in battery-powered radios, but also in a variety of other wearable devices. In Fig. 3.7‑12, 3.7‑13, 3.7‑14 show several diagrams of such indicators. All of them work according to the already described pulse principle and are essentially economical pulse generators loaded onto an LED. The generation frequency in such circuits is chosen quite low, in fact, at the border of visual perception, when the blinking of the LED begins to be clearly perceived by the human eye.

Rice. 3.7-12. Economical LED indicator based on a unijunction transistor

Rice. 3.7-13. Economical LED indicator based on unijunction and bipolar transistors

Rice. 3.7-14. Economical LED indicator based on two bipolar transistors

In VHF FM receivers, three LEDs can be used to indicate tuning. To control such an indicator, a signal is used from the output of the FM detector, in which the constant component is positive for a slight detuning in one direction from the station frequency and negative for a slight detuning in the other direction. In Fig. Figure 3.7-15 shows a diagram of a simple setting indicator that works according to the described principle. If the voltage at the indicator input is close to zero, then all transistors are closed and the LEDs \(HL1\) and \(HL2\) do not emit, and through \(HL3\) a current flows, determined by the supply voltage and the resistance of resistors \(R4 \) and \(R5\). With the ratings indicated in the diagram, it is approximately equal to 20 mA. As soon as a voltage exceeding 0.5 V appears at the indicator input, the transistor \(VT1\) opens and the LED \(HL1\) turns on. At the same time, the transistor \(VT3\\) opens, it bypasses the LED \(HL3\), and it goes out. If the input voltage is negative, but the absolute value is greater than 0.5 V, then the LED \(HL2\) turns on, and \(HL3\) turns off.

Rice. 3.7-15. Tuning indicator for VHF-FM receiver on three LEDs

A diagram of another version of a simple fine-tuning indicator for a VHF FM receiver is shown in Fig. 3.7-16.

Rice. 3.7-16. Tuning indicator for VHF FM receiver (option 2)

In tape recorders, low-frequency amplifiers, equalizers, etc. LED signal level indicators are used. The number of levels indicated by such indicators can vary from one or two (i.e. control of the “signal present - no signal” type) to several dozen.

The diagram of a two-level two-channel signal level indicator is shown in Fig. 3.7‑17. Each of the cells \(A1\), \(A2\) is made on two transistors of different structures. If there is no signal at the input, both transistors of the cells are closed, so the LEDs \(HL1\), \(HL2\) do not light up. The device remains in this state until the amplitude of the positive half-wave of the controlled signal exceeds by approximately 0.6 V the constant voltage at the emitter of the transistor \(VT1\) in the cell \(A1\), specified by the divider \(R2\), \ (R3\). As soon as this happens, the transistor \(VT1\) will begin to open, a current will appear in the collector circuit, and since it is at the same time the current of the emitter junction of the transistor \(VT2\), the transistor \(VT2\) will also begin to open. An increasing voltage drop across the resistor \(R6\) and LED \(HL1\) will lead to an increase in the base current of the transistor \(VT1\), and it will open even more. As a result, very soon both transistors will be completely open and the LED \(HL1\) will turn on. With a further increase in the amplitude of the input signal, a similar process occurs in cell \(A2\), after which the LED \(HL2\) lights up. As the signal level decreases below the set response thresholds, the cells return to their original state, the LEDs go out (first \(HL2\), then \(HL1\)). The hysteresis does not exceed 0.1 V. With the resistance values ​​​​indicated in the circuit, cell \(A1\) is triggered at an input signal amplitude of approximately 1.4 V, cell \(A2\) - 2 V.

Rice. 3.7-17. Two-channel signal level indicator

A multichannel level indicator on logical elements is shown in Fig. 3.7‑18. Such an indicator can be used, for example, in a low-frequency amplifier (by organizing a light scale from a number of indicator LEDs). The input voltage range of this device can vary from 0.3 to 20 V. To control each LED, an \(RS\)-trigger assembled on 2I-NOT elements is used. The response thresholds of these triggers are set by resistors \(R2\), \(R4-R16\). A LED extinguishing pulse should be periodically applied to the “reset” line (it would be reasonable to supply such a pulse with a frequency of 0.2...0.5 s).

Rice. 3.7-18. Multichannel low-frequency signal level indicator on \(RS\)-triggers

The above circuits of level indicators ensured sharp response of each indication channel (i.e., the LED in them either glows with a given brightness mode or is turned off). In scale indicators (a line of sequentially triggered LEDs), this mode of operation is not at all necessary. Therefore, simpler circuits can be used for these devices, in which the LEDs are controlled not separately for each channel, but jointly. The sequential switching on of a number of LEDs as the input signal level increases is achieved by sequential switching on voltage dividers (on resistors or other elements). In such circuits, the brightness of the LEDs gradually increases as the input signal level increases. In this case, for each LED, its own current mode is set, such that the glow of the specified LED is visually observed only when the input signal reaches the appropriate level (with a further increase in the input signal level, the LED lights up more and more brightly, but up to a certain limit). The simplest version of an indicator operating according to the described principle is shown in Fig. 3.7-19.

Rice. 3.7-19. Simple LF signal level indicator

If it is necessary to increase the number of indication levels and increase the linearity of the indicator, the LED switching circuit must be slightly changed. For example, an indicator according to the diagram in Fig. 3.7-20. It, among other things, has a fairly sensitive input amplifier that provides operation both from a constant voltage source and from an audio frequency signal (in this case, the indicator is controlled only by the positive half-waves of the input alternating voltage).