A light telephone based on a laser pointer. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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A light telephone based on a laser pointer. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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The possibility of using a laser pointer to transmit AF signals over a certain distance is due to the fact that the power of its radiation depends on the value of the supply voltage. Therefore, when the voltage changes in time with the speech signal, amplitude modulation is obtained. If the pointer beam is directed to the receiver of the subscriber, in which the photosensor with the amplifier is installed, a sound will be heard in the dynamic head of the receiver. Two transceivers located at communication points form a light telephone. The scheme of one transceiver is shown in fig. 1.

(click to enlarge)

The laser pointer is not altered, but only connected to the electronic "stuffing" of the device, and the case is connected to the power plus. The device consists of transmitting and receiving nodes, which are structurally placed in the handset (except for the pointer and phototransistor). Power comes from a stand-alone or network unit.

The light telephone has three modes of operation: "On duty", "Call", "Work". In the first mode, the transmitting node is de-energized and only the receiving node works. In the second mode, the transmitting node is turned on and a tone signal is given to the subscriber. After the subscriber answers, the third mode is switched on, while both nodes work and the conversation is carried on, as on a regular phone.

The receiving node is made on a DA1 chip, which is an AF amplifier. A photodetector on a phototransistor VT1 is connected to the input of the amplifier. The signal from the subscriber's laser pointer that hits it is amplified and fed to the BF1 telephone capsule located in the handset.

After the supply voltage is applied, the receiving unit works constantly, its sensitivity can be adjusted by the tuning resistor R2.

The transmitting node is made on the same "amplifying" chip (DA2). At the input of the amplifier, the BM1 microphone is turned on, and its output is connected through a current-limiting resistor R13 with "its" pointer. Zener diode VD1 protects the pointer from high voltage and is closed during normal operation.

When the AF signal is applied, the current through the resistor R13 and the pointer will begin to change in time with the change in the signal amplitude, i.e. the radiation power will be modulated by the signal.

After applying the supply voltage, the transmitting node is de-energized. It will start working only after pressing the SB1 "Call" button or when the contacts of the switch SA1 "Work" are closed. If the button is pressed, a supply voltage is supplied to the node, at the same time, its contacts SB1.2 turn on the positive feedback circuit C7R7. The amplifier turns into a generator operating at a frequency of about 1000 Hz. A call tone is transmitted through the pointer. At the same time, the SB1.1 contacts disconnect the BF1 capsule from the receiving node and connect it through the resistor R6 to the output of the DA2 microcircuit. A call signal is heard in the capsule, indicating that it has been sent to the pointer. The volume of the signal is set by selecting the resistor R6.

As soon as the subscriber's answer is heard, switch SA1 switches the device to the "Operation" mode. At the end of the connection, the switch is set to its original position, shown in the diagram.

Instead of these microcircuits, imported TDA2003 or similar ones are suitable, and the phototransistor will completely replace the photodiode connected by the anode to the common wire. The zener diode should first be selected with a stabilization voltage of 4,6 ... 4,7 V. Oxide capacitors - K50-6, K50-16, the rest - K10-17, KLS or similar. Trimmer resistors - SPZ-19, constants - MLT, S2-33. The switch and the button any small-sized. The capsule (with a resistance of 30 ... 100 Ohms) can be either small-sized from headphones or from a handset. Microphone - electret MKE-332 or similar imported.

Most of the parts (except for the phototransistor and pointer) are placed inside the handset (Fig. 2), with the switch, button, microphone and capsule mounted on the body of the handset, and the C7R7 chain is mounted on the button.

Light telephone based on a laser pointer

The rest of the parts are mounted on boards (Fig. 3 and 4) made of one-sided foil fiberglass.

Light telephone based on a laser pointer

The board of the transmitting node is installed at the bottom of the tube, and the receiving node - at the top (Fig. 5).

Light telephone based on a laser pointer

The phototransistor is placed in an opaque tube made of insulating material with an inner diameter of 10 ... 15 and a length of 40 ... 50 mm - it protects the phototransistor from interference (sunlight, lighting devices).

In order not to alter the pointer and, if necessary, use it for its intended purpose, it should be inserted into a tube with an inner diameter that is 1 ... 1.5 mm larger than the pointer diameter. Then, when a pointer is inserted into the tube, its button will be in the pressed state. But first you need to connect to the pointer (by clamps or "cold soldering" - by screwing the ends of the conductors) a two-wire cord coming from the transmitting node.

Setting up the device begins by temporarily disconnecting the C7R7 chain and pointer. Both nodes are turned on and the operability of the microcircuits is checked by measuring the voltage at their outputs - it should be equal to about half the supply voltage. On the phototransistor and microphone, the voltage should be within 4 ... 8 V.

By pressing the button further and talking in front of the microphone, you will hear a loud and clear sound in the capsule. In the upper position of the slider R9 according to the scheme, self-excitation is possible due to acoustic feedback.

After releasing the button, direct the phototransistor to the switched on lighting lamp. An alternating current background should be heard in the capsule.

After that, the C7R7 chain is installed and, by selecting its parts, the required ring tone is obtained. Connect the pointer and control the voltage on it. By selecting the resistor R13, the voltage is 4 V.

The laser beam is aimed at a light object installed on the table, and then a phototransistor is directed to the light spot. When talking in front of the microphone, the sound in the capsule should be heard. Resistors R2 and R9 set the sensitivity of the nodes to avoid self-excitation, and the sound was as loud as possible and without distortion.

The second device is set up in the same way, and experimental communication is carried out at a distance of several meters, directing the laser beam to the subscriber's phototransistor. It is possible that the power of the laser radiation will be high. In this case, a light-absorbing shutter will have to be placed in front of the phototransistor. If the connection is good, you can conduct experiments at a greater distance.

In practice, the communication range can reach several hundred meters, but within the line of sight. True, it will be necessary to accurately orient the laser beam and securely fix the position of the pointer and phototransistor. This adjustment should be carried out at night, using a telescope or binoculars.

Remember that when setting up the device and its operation, it is strictly forbidden to direct the pointer beam at the eyes - this is dangerous.

Author: I. Nechaev, Kursk

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