Old motherboards of computers, the use of which is no longer relevant, can be used as "donors" of parts. So, for example, from there you can take field-effect transistors (with power characteristics of the order of 20-30 volts / 30-70 amps!), oxide or solid state electrolytic capacitors and chokes on the circuit nutrition.
Chokes are designed to filter the high-frequency component in the power circuit and are several turns of copper wire wound on ferrite rings. You can use them for their intended purpose, in the output circuits of power supplies. But, in addition, you can use the rings themselves for self-production of not complicated, but useful circuits for the radio amateur. Below will be presented two such schemes, which have been collected in practice more than once and have shown good repeatability, "loyalty" to the elements used and reliability in operation.
1. ESR meter
It is a device for measuring the equivalent series resistance (ESR or ESR) of electrolytic capacitors at high frequencies. With such a device, you can easily and quickly check the performance and quality of capacitors (for example, on the same motherboards). In this case, the capacitors can not be desoldered, but checked directly on the boards (of course, de-energized). The device is not afraid of the residual charge of the capacitor (except for capacitors with capacities of more than 5000 μF or high-voltage ones) and does not require observing the correct polarity of the connection during measurements. This factor greatly simplifies the measurement process.
The tested capacitor is connected to probes X1 and X2. In this case, a signal with a frequency of about 50... 60 kHz begins to be generated in the winding I. Depending on the state of the tested capacitor, the amplitude of this signal will have a certain level. When the power is turned on and the contacts of the X1 and X2 probes are open, the HL1 LED will light up.
If the probes now touch the leads of a good, serviceable capacitor (as already mentioned, the polarity does not matter), the LED should completely go out. The performance of this meter can be easily checked by short-circuiting the probes.
The LED should also go out in this case. With a "bad" capacitor with a high ESR value, the LED will continue to light up with a brightness corresponding to its resistance value.
Almost any low-power transistor of the N-P-N structure can be used in the circuit, the resistor R2 should be a power of 2 watts (it limits the discharge current of the tested capacitor), resistor R1 - any power.
The transformer is wound on a ferrite ring. The ring can be of any size sufficient to wind all of its windings. The generator winding consists of 60 turns of wire of the PEL type 0.2... 0.4 with a branch from the middle of the winding (that is, 30 + 30 turns), the "measuring" winding (where the resistor R1 and probes) - 3-4 turns of the PEL wire 1.0. The "indication" winding should ensure the normal brightness of the LED and contains about 6 turns of PEL wire 0.2... 0.4. The exact number of turns can be selected experimentally, depending on the type of LED used, according to the maximum brightness of its glow.
The circuit is powered by a battery or accumulator with a voltage of 1.2... 1.5 volts.
2. DC voltage converter 1.5 - 9 volts
This simple device allows you to increase the voltage value from 1.5... 3 volts (for example, penlight batteries) to the higher value you need (5, 10, 12 volts and more).
Transistors can be used with any P-N-P structure and power, depending on the required output current value (in the load). For example, for a load current of no more than 100 mA, transistors such as KT203, KT208, KT501 and others are suitable. In this case, you should choose transistors with a permissible base-emitter voltage of at least 10 volts and copies with the closest possible parameters should be used in pairs.
Winding I consists of 10... 20 turns of 0.2 mm PEL type wire with a branch from the middle of the winding, winding II - 70 turns of the same wire and also with a branch from the middle. First, winding II should be wound, and then winding I should be wound on top of it. This will allow, by selecting the exact number of turns of the winding I, to set the voltage value you need at the output. At the output, we get a constant voltage (without the use of additional diode rectifier). Capacitor C1 serves to smooth out the high-frequency ripple of the converter output voltage, and resistor R1 acts as a low-power load. The capacity of the capacitor C1, if necessary, can be slightly increased (up to 100 μF), its operating voltage must correspond to the output voltage of the converter (must be higher than this value). When the converter is operating on a permanently connected load, the resistor R1 can be excluded from the circuit.
In addition to the simplicity of the circuit, a useful feature of such a converter is also the fact that when the load is off, it does not consumes current from the power source (its value is less than the self-discharge current of the battery) and does not require the installation of a separate switch.