Pen-flashlight. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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The proposed device (automatic pocket illuminator - OKA) is a "writing instrument" (pen), supplemented by a flashlight on a high-brightness LED and placed in the body of the marker. Turning on and off the LED is carried out automatically using a photo relay.

The device is equipped with a clamping bracket for fastening in the outer pocket of a jacket, jacket or work overalls. The flashlight serves as an individual means of local lighting and is designed to automatically turn on emergency lighting. Automatic turning on of the device is especially important if, when the light goes out, a person is near dangerous objects and his hands are busy (it is difficult to quickly turn on a hand flashlight).

Another option for using the device is to embed it in a badge to highlight the symbol depicted on it. Somewhere in a disco, such an icon will effectively turn on when the general lighting is turned off. In standby mode, OKA is very economical. It consumes current. comparable to the self-discharge current of the power source, so you need to turn off the power only after the end of using the device.

The illuminator (Fig. 1) consists of a photo relay, a current switch and the actual emitter HL1 (high brightness LED). The photo relay is assembled on the elements R1 ... R4, DA1, VT1, the key is on a field effect transistor VT2. A current-limiting resistor R1 is connected in series with the emitter HL5.

When the contacts of the switch SA1 are closed, power is supplied from the battery GB1. If the phototransistor VT1 is lit, the device is in standby mode. The divider R1-R2 sets a reference voltage equal to half the supply voltage at the non-inverting input (pin 3) DA1. Resistor R3 and collector-emitter junction T1 form a second voltage divider, the signal from the midpoint of which is fed to the inverting input (pin 2) DA1. The voltage drop across the divider elements is directly proportional to their resistances. Operational amplifier DA1 is switched on in comparator mode (without feedback resistor). The comparator compares the voltages at its inputs and outputs the result of the comparison as a low or high voltage level at the output. If the voltage at the direct (non-inverting) input is greater than at the inverting one, the voltage at the output of the op-amp tends to + Un, on the contrary, to the potential of the common wire (ground).

When the phototransistor VT1 is illuminated, the resistance of its collector-emitter junction (compared to R3) is small, and almost all the voltage of GB1 drops across the resistor R3. Therefore, at the output (pin 6) DA1 voltage is close to ground. For a darkened 4-way transistor, the junction resistance increases, and the voltage at the inverting input becomes less than 0,5Un (determined by the current through the VT1 junction). Therefore, at the output (pin 6) DA1, the voltage becomes close to the supply voltage and opens the transistor VT2. The channel resistance (source-drain) VT2 decreases to a few ohms, and the current flowing through the channel enters the load, i.e. LED HL1 glows brightly. Resistor R5 limits the current flowing in the load to the rated current HL1 (20 mA). Key VT2 is open until then. while the phototransistor VT1 is dark. Resistor R4 sets the control current of the programmable op-amp DA1.

Assembled without errors and from serviceable elements, the device starts working immediately after turning on the power. You can reduce the sensitivity of the o\u3e photorelay by reducing the resistance R1 to 1 MΩ. You can correct the current through the HL5 LED by selecting R1. In OKA, OMLT resistors are used, capacitor C4 is ceramic. KW1. Switch SA102 - SMTS-140 (especially miniature) or sliding, for example, from an old calculator. Op-amp - K12UD140 (KR1208UD1) with low current consumption and low supply voltage, so replacing DA1 with other op-amps is impractical. The phototransistor VT5323 is taken from the ES01-1 computer repair kit. It can be replaced (with some loss of sensitivity o\u1e photorelay) with a photoresistor FSD-XNUMX. If there are several types of photoresistors, photoresistors with a high dark resistance should be preferred. As HLXNUMX, you should use super-bright green, white or blue LEDs.

The device is placed on a printed circuit board, the drawing of which is shown in Fig.2.

The diameter of the holes on the board for the microcircuit is 0.7 ... 0.8 mm. the rest - 0.8 ... 1 mm. To reduce the size, the OKA board does not have mounting holes and is installed in the case on friction. The print design can be done by thermal transfer [1] or translated with a carbon paper and then circled with an acid-resistant permanent marker (for example, "Centro pen 2846 CE PERMANENT") or a felt-tip pen for signing CDs. Soldering components during installation should be carried out with a low-voltage or grounded soldering iron.

The board is located in the case of a thick marker with a bevel-clothespin for carrying in a pocket (Fig. 3). LED HL1 in the vertical position of the device should "look forward".

The case can be homemade. Its dimensions are determined mainly by the size of the used batteries or batteries. As GB1 can be used, for example. 3-4 accumulators D-0.26D (025 mm) or more compact foreign-made. The mounting location of the SA1 switch is determined by the shape of the OKA case. and the length of the segment of the writing rod - the height of the body. If the ML1 LED is made retractable with the possibility of tilting at an angle of 90 °, then OKA will turn into a writing pen with illumination for writing in the dark. To avoid complications, you can simply install the connector for the HL1 on the OKA case.

LEDs with different lead shapes are inserted into replaceable caps and filled with epoxy resin (Fig. 4).

The OKA circuit can also be used as an economical photorelay, switching on any 4.5-volt load (receiver, radio microphone, various sound effects). The polarity load is turned on instead of the HL1 LED. Resistor R5 is excluded. Field-effect transistor KP501A provides DC switching in load up to 180 mA.

Literature

1. A. Oznobikhin. VHF receiver "1066-R". - Radioworld. 2007. No. 8. P.41.

Author: A. Oznobikhin, Irkutsk

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