"To help for radioamateur", issue 106 (VRL 106).
If you look at Figure 1, you'll notice that the transistors are connected in such a way that it forms an analogue of thyristor. The control electrode of "thyristor" is connected to the inductor L1, the electrode circuit is connected to the resistor R1. The transistor VT1 can be silicon and germanium, the transistor VT2 should be germanium only, because it has the ability to amplify signals without bias voltage on the base.
Fig. 1. Circuit diagram of RL Relaxation oscillator.
Principe of operation. When power is first applied to the circuit, the transistor VT1 starts to turn on (it becomes active), because the initial collector current of the transistor VT2 flows through emitter junction of the transistor VT1. Since the oscillator is a non-inverting amplifier, its output is connected to the input. A random increase in the collector current of any of the transistors (for example, because of the intrinsic noise of transistors, external interference, etc.) will be immediately amplified and it leads to an avalanche turning on the transistors to the saturation state, as usually happens in trinistor (time t1 in Fig. 2).
Fig. 2. Oscillogram of RL Relaxation oscillator.
The inductor coil does not prevent from opening transistors, because its resistance to the pulse signals is too high.
After switching on the the transistors, the current through the coil increases exponentially. The collector current of transistor VT1 will be increased also. Very soon the transistor VT1 goes out of saturation, and voltage across it increases. The voltage across the coil will decreases, and develops an avalanche process if switching off transistors (time t 2 in Fig. 2).
The energy stored in the coil in a magnetic field prevents the rapid decrease in current through the coil, and the current gradually decreases to zero. This current is supported by self-induction EMF, whose value after the switching off of transistors may be in ten times greater than the supply voltage. Energy of magnetic field is dissipated as heat in the junctions of transistors, self-induction EMF gradually decreases to zero, the current through the coil stops and the cycle of oscillation is repeated (time t3 in Fig. 2).
Thus, on the inductor L1 we see a continuous sequence of rectangular pulses of voltage, and the current flows through the coil as a sequence of sawtooth pulses.
The process of oscillating occurs slightly different when the winding of earphones BF1 used as a coil (Fig. 3).
Fig. 3. Circuit diagram of RL Relaxation oscillator with the headphones. Both transistors are germanium.
The pulse repetition rate on earphones BF1 is synchronized with the intrinsic resonant frequency of the membrane. (Fig. 4).
Fig. 4 Oscillogram of the signal on the headphones.
This happens because the headphone is a reversible converter, i.e., vibrations of the membrane caused by external voltage pulses will excite in the winding of headphone the AC voltage (dashed line in Fig. 4), which is summed with the voltage of the oscillator and applied to the base of the transistor VT2.
The work of headphones at the resonant frequency of the membrane dramatically increases the efficiency of the oscillator as elector acoustic transducer, resulting in significant sound volume at low power consumption from the power source.
To make timbre of headphones softer, the capacitor C1 is connected to the windings of headphones - in this case the form of oscillations will be almost sinusoidal, and the impulses of self-induction EMF is practically disappear (by the way, this will eliminate the possibility of the breakdown of transistors).
Oscillator, shown in Fig. 3, can be used as an economical and simple sound alarm, especially in battery-powered devices, as well as a probe to test ("continuity") of various electrical circuits. In the latter case the test probes should be connected in series with power supply and.
On the place of BF1 can be used headphones or earpiece with resistance less then 250 ohms. Transistor VT1 - MP35 - MP38 (NPN), and VT2 - MP21, MP25, MP26 (PNP) (it's Russian germanium transistors).
Figure 5 shows the circuit diagram with a loudspeaker BA1. The circuit is adjusted with the variable resistor R1, which changes the mode of stable oscillations. The sound of a low-power loudspeaker (0.5GD-30 or similar) with a small area of the diffuser sounds like a car horn.
Fig. 5. Circuit diagram of RL Relaxation oscillator with the loudspeaker.
This oscillator can be used in models of cars, as the apartment bell or the alarm bell, or like a horn on a bicycle. Transistors of the oscillator can be the same as in the previous case. Transistor VT2 can be PNP transistor like MP39-MP42.
If the collector of transistor VT1 and VT2 base connect with the coupling capacitor C2 and use the headphone BF1 (Fig. 6), the oscillator will produce packets of pulses that mimic the bird's trill. Capacitors can be any type, variable resistor - SP-1, headphone (or earpiece) - with resistance less than 250 ohms, for example, DEM-4M, transistors - the same as in the previous circuit.
Fig. 6. Circuit diagram of RL Relaxation oscillator with coupling capacitor.
As mentioned above, the amplitude of the pulse self-induction EMF in the winding of headphone reaches a significant magnitude. By using this, you can use the oscillator as a DC to DC voltage converter. This converter, for example, will become a power source for an analog multimeter when it is measuring high resistance. Most of analog multimeters required a single source for this mode, which is not always at hand. Furthermore, additional operations related to its commutation, reduce the efficiency of measurements. All this leads to the fact that one of the ranges remains unused.
In this situation can help a voltage converter made of RL oscillator (Fig. 7), embedded in the body of analog multimeter. It contains the minimum elements and does not require any adjustment. In the design of analog multimeter it is necessary to introduce small changes: install a switch SA1 and connect its contacts to the positive power wire and to the negative terminal of the element G1.
Fig. 7 Circuit diagram of DC-DC converter.
When the switch is off then it shunts the circuit and analog multimeter is using in normal mode. Although the converter at the same time remains connected to the power supply, it consumes almost no energy - diode VD2 and Zener diode VD1 is connected in relation to a power source in the opposite direction.
For measuring high resistances the switch SA1 should be turned to "ON." position. Then the oscillator based on transistors VT1, VT2 starts to oscillate. Pulses of self-induction EMF of headphones winding BF1 will charge the capacitor C1 through the diode VD2. The voltage across this capacitor increases rapidly and stabilized at approximately 12 V (the breakdown voltage of Zener diode VD1). Summed with the power supply voltage, it reaches the measuring circuit of the analog multimeter. Sound signal emitted by headphones BF1, reminds that after the measurements power should be turned off.
The current consumption by the converter from a voltage source is approximately 5 mA. The maximum current load of the converter must not exceed 100 μA, because the amplitude of the output voltage pulses will be increased. Therefore, it is desirable to apply the converter only to analog multimeter with a minimum measurement of direct current of 100 μA.
Transistors of this converter can be the same as in previous devices. In this circuit can be used zener diodes D813, KS213, diodes D219, D223, KD102, KD103 with any letter index. Resistor - any type, small-sized. Headphones - headphone capsule DEM-4M, TK-67 or other electromagnetic system with the resistance of the winding 50...100 ohms.