Economical indicator

This article describes a simple and economical electrical switch location indicator.

Most often, optical indicators are built into those power switches, which are located in a darkened part of the apartment, to accurately indicate their location in the dark.

Usually, the optical network indicator circuit is connected in parallel with the contacts of the existing network switch, whence follows, that the indicator consumes some power from the mains always, when the breaker is open, and this is most of the time of the day, therefore, the need for its economy becomes obvious.

Sufficiently economical, but not the brightest are emitters on gas-filled elements (current consumption about 1 mA, with a current limiting resistor of about 120 k). Besides, they do not have a very large resource. The most striking, but less economical (consumption current 5…20 mA) are LED indicators, which have a much greater resource.

In order to increase the efficiency of network LED indicators, they are transferred to a dynamic mode of operation.. This mode allows you to significantly reduce the average current, consumed by the indicator from the network, and also increase the amplitude of the current pulse through the LED, that is, get flashes of higher brightness.

It should be noted, what is the frequency of flares (at RC = const) generator also carries additional information about the supply voltage, moreover, with full preservation of the main everyday function of the indicator.

Network indicator circuit, which is designed to be built into mains switches, shown in Fig.1. As can be seen from Figure 1, this is a relaxation generator, in which the diode VD1 serves as a half-wave mains voltage rectifier. Resistor R1 has two functions – serves as a current-limiting and time-setting element of the relaxation generator at the same time, and capacitor C1 serves as a storage element.

rice. 1

Active element of the generator – planar transistor VT1 structure n-p-n, which is included inversely (on the emitter "+" supply voltage) bipolar, and operates in the reversible avalanche breakdown mode. When operating in this mode, there is a section with negative resistance on the I - V characteristic of transistor VT1, therefore, the introduction of positive feedback from the input to the output of the generator is not required.

The oscillation frequency (outbreaks) determined by the parameters of the R1C1 circuit, and also the value of the supply voltage. Flash energy E = CU2/2 (J.). In the author's version of network indicators, the flash frequency is chosen equal 1 …2 Hz.

As known, a feature of turning on the transistor in this mode is also a higher, than with a normal non-inverse connection, temperature stability of the I - V characteristic. So, For example, for KT315 transistors, the breakdown voltage changes by only 100…150 mV, when the temperature changes from -50 up to + 65 ° C.

This network indicator circuit, compared to indicators on gas-filled appliances, has one more advantage: when installed in the power switch, they were not "highlighted", as a basement compact fluorescent lamp MAXSUS spiral 20W 4100K, and a homemade lamp based on an LED block of the R58-60LED brand 3528 4W 250 lm, with two groups connected in parallel, composed of 30 ultra-bright SMD LEDs connected in series, ballast capacitor 0.82 uF, bridge rectifier and filter capacitor 2.2 uF 250 AT.

The physical meaning of this phenomenon is, that although the supply voltage with an open contact of the mains switch also flows through the indicator circuit to the said lamps. However, the value of the current in the circuit is so small, which does not exceed the current loss in the lamp circuit, therefore, the accumulation of energy on the capacitors of the mains filter of these lamps does not occur, and their emitters are not "highlighted".


The connection of the indicator parts is carried out by volumetric installation by connecting the leads of the parts to each other along the shortest distance in the form of compact blocks in two design options.

First option (Fig. 2) designed for installation in an opaque enclosure, with mechanical modification of the case (drilling a hole for the LED), therefore, the LED is placed on two conductors of the required length from the volume of the block.

rice. 2

The second option (Cm. a photo at the outset) designed for installation in a transparent housing of a power switch without mechanical modification, therefore the LED is located in the block itself.


Transistor VT 1 – KT315 with any letter index, or imported, 2SC3370, 2SC945, VS337, 2SC337-40, 2SD667, BD667.

diode VD1 – any silicon with reverse voltage from 400 B and small overall dimensions.

VD2 LED – conventional indicator type ALL307, red or green glow color, the brightness of which is quite sufficient when installed in switches, which are located in the living area.

If the power switch is far enough away, and in place, which can be seen, but it is inconvenient to get to it, Super-bright LED can be installed in the indicator circuit, whose flashes are clearly distinguishable from a sufficiently large distance.

Condenser C1 with low leakage type ETO or K52-2, K52-1, K52-3, K50-35, or similar imported.

The author's copy is equipped with an imported oxide capacitor of vertical design 100 uF 25 AT.


Since the planar transistors KT315 as elements are not intended (although they can work) by manufacturer for avalanche operation, in some cases it may be necessary to select a transistor.

Considering the presence of mains voltage on the elements of the device, transistor preselection, and sometimes a capacitor, for the indicator it is better to hold, using a network low voltage power supply (IP) with smoothly varying output voltage from 0 to 30 AT, For example, type B5-7. The circuit for the pre-selection of transistors is shown in rice.3.

rice. 3

For the convenience of selecting transistors, operating in the reversible avalanche breakdown mode with inverse power-on, out of stock, assemble test circuit. A socket for installing microcircuits is used as XS2 female connectors. The result is a test bench, in which the replacement of the transistor is carried out by simply installing it in the socket.

The selection of transistors is carried out as follows. Voltage is applied to the assembled circuit from the IP 3 AT. The LED should not light up. Further, the value of the supply voltage is gradually increased until stable generation with a frequency of about 1…2 Hz. Flash and LED off should be clear and "not blurry", which indicates the good switching properties of this instance of the transistor.

The transistor selected in this way is installed in the circuit of Fig. 1, temporarily replacing resistor R1 with a chain, consisting of series-connected avometers in alternating current measurement mode, constant resistor 270 kohm and variable resistor 2 my, entered at the maximum resistance value.

By changing the position of the moving contact of the variable resistor in the direction of decreasing resistance, approximately the same generation frequency is achieved, as when testing with a power supply on the bench; while controlling the resulting current value, which should be in the interval 0.1 …0.4 mA.

It should be noted, that when the mains switch is opened, the LED flashes do not appear immediately, and with some delay, what is not a malfunction, but it is caused by a sufficiently large time constant of the circuit itself and the formation of the storage capacitor.

Statistical tests

Statistical tests, conducted by me at the previously mentioned stand, show, that from the batch of available transistors KT315 (used and new) in quantity 50 PC. steadily generated at supply voltage 9 AT – 15 PC., at 12 AT – 18 PC., at 15 AT – 10 PC., at 18 AT – 5 PC., (a) 2 PC. did not generate at all.

At the same stand, single copies of imported n-p-n conduction transistors were tested., types 2SC3370, 2SC945, BC337, 2SC337-40, 2SD667, BD667, available, which on the voltage across the capacitor C1 (2…3 AT, Figure 3) did not differ from transistors KT315.

One more possibility of the relaxation oscillator circuit should also be noted, на­пример, for transistors, which generated at supply voltage 9 AT: with increasing supply voltage, the frequency of flashes increased, visually turning into a constant glow.

Additional measurements, carried out on the capacitor C1 (Figure 3) using an oscilloscope, show, that with an increase in the voltage applied to the circuit, the frequency of the relaxation generator increases, and the amplitude decreases. Limiting frequency of generation (breakdown of vibrations) reached 100…200 kHz depending on the transistor instance.

Author: Sergey Elkin, g. Zhytomyr
A source: Amateur radio №9-10 / 2020

Leave a Reply

Your email address will not be published.