When no great accuracy is required to measure DC voltage, I use all kinds of light and sound indicators. Popular called "probes-screwdrivers for the control of DC voltage in the car (12. ..24 V) with indicators based on LEDs.
Offer your circuit tester on non-deficient operational amplifiers COD (with field-effect transistors on the inlet and, respectively, with a high input impedance). Scope unit - automotive engineering, power supplies, home lab ham, etc., where control of DC voltage in the range 5...20 V.
In the scheme implemented two of the same type of node, the comparison voltage. Reference level is the voltage across the Zener diode VD1. The device is powered from a controlled voltage. Indication of voltage levels is performed by two LEDs (preferably different colors). But the number of levels of control can easily expand by connecting similarly some similar OS. Instead LEDs HL1 and HL2 as audio indicators you can apply the primer with built-in generators.
If none of the LEDs HL1, HL2 is not lit, then the measured voltage below 10 V. When the glow only HL1 - voltages 12...13, when the glow only HL2 voltage is in the range of 13.5 15... 8. If both LEDs are lit or flicker, voltage controlled more 16.5 V.
All fixed resistors - type MPT-0,25, MF-25, turn - multi-turn, SP5-1VB, SP5-2, SDR-19 or equivalent with linear characteristic resistance. Capacitors - CMB. The Zener diode VD1 is replaced by XA, XB. The LEDs can be replaced by L-239EGW, CIPAV-M or similar.
The building site is to install the trigger threshold for each led in individually trimmer resistors R4 and R7. As the power source (and controlled voltage) at the time used to establish adjustable a regulated source of voltage 5...20 V.
Before switching on the engines of trimming resistors output in Average position. On the power supply set the voltage of 12 V. connect reasonably accurate voltmeter (multimeter) in the continuous measurement mode voltage. Adjustable resistance R4 achieve emission LEDs HL1. Increase the voltage to 13.5...14 set by resistor R7 lit HL2. Reduce the voltage to 10 V and make sure the LEDs are not brings one into.
Author: A. Kashkarov, G. S. Petersburg