Description.
The circuit diagram of a simple capacitance meter using IC LM2917 is shown here. The LM2917 is a high gain monolithic frequency to voltage converter IC from National Semiconductors. Even though the main application of LM2917 is in tachometers, it can be also used for a variety of applications like this.
Capacitance values from 0.01uF to 0.1uF can be measured using this circuit. The capacitance to be measured is connected between pin 2 of the IC and ground. The output voltage available across resistor R2 will be proportional to the Cx and it will be displayed by the meter. Resistor R1 can be used for calibrating the circuit.
Circuit diagram.

Notes.

• The circuit can be assembled on a vero board.
• Resistor R1 can be used for calibration.
• Meter M1 should be a 10V FSD voltmeter.
• The IC must be mounted on a holder.

Description.
LM331 is basically a precision voltage to frequency converter from National Semiconductors. The IC has a hand full of applications like analog to digital conversion, long term integration, voltage to frequency conversion, frequency to voltage conversion. Wide dynamic range and excellent linearity makes the IC well suitable for the applications mentioned above.
Here the LM331 is wired as a frequency to voltage converter which converts the input frequency into a proportional voltage which is extremely linear to the input frequency. The frequency to voltage conversion is attained by differentiating the input frequency using capacitor C3 and resistor R7 and feeding the resultant pulse train to the pin6 (threshold) of the IC. The negative going edge of the resultant pulse train at pin6 makes the built-in comparator circuit to trigger the timer circuit. At any instant, the current flowing out of the current output pin (pin 6) will be proportional to the input frequency and value of the timing components (R1 and C1). As a result a voltage (Vout) proportional to the input frequency (Fin) will be available across the load resistor R4.

Circuit diagram.

Notes.

• The circuit can be assembled on a vero board.
• I used 15V DC as the supply voltage (+Vs) while testing the circuit.
• The LM331 can be operated from anything between 5 to 30V DC.
• The value of R3 depends on the supply voltage and the equation is R3= (Vs – 2V)/ (2mA).
• According to the equation, for Vs = 15V, R3=68K.
• The output voltage depends on the equation, Vout = ((R4)/(R5+R6))*R1C1*2.09V*Fin.
• POT R6 can be used for calibrating the circuit.

Description.

Here is the circuit of a very simple transistor tester which used two LEDs for displaying the condition of a transistor. Both PNP as well as NPN transistors can be tested using this circuit. Quad 2 input CMOS NAND gate IC CD4011B is the heart of the circuit. Out of the four NAND gates inside the IC, only three are used here and they are used as NOT gates by shorting their input terminals. Gates U1a, U1b, resistor R1 and capacitor C1 forms a square wave oscillator. The frequency of this oscillator can be adjusted by using R1. The output of the oscillator is inverted using the gate U1c. The inverted oscillator output is connected to the base of the transistor under test through the resistor R2 and the non inverted oscillator output is connected to the emitter of the transistor under test using the resistor R3.

The status of the LEDs D1 and D2 reveals the condition of the transistor under test. If red LED is ON, It indicates that the transistor under test is a good NPN. If green LED is ON, it indicates that the transistor under test is a good PNP. If both LEDs are ON, it indicates that the transistor under test is short. If both LEDs are OFF, it indicates that either the transistor is bad or you may have connected it incorrectly.

Circuit diagram.

Notes.

• The circuit can be easily assembled on a Vero board.
• Use 5V DC for powering the circuit.
• All unused pins of the IC must be connected to ground.

Description.
The circuit given here is of a very useful and accurate digital voltmeter with LED display using the ICL7107 from Intersil. The ICL7107 is a high performance, low power, 3.5 digit analog to digital converter. The IC includes internal circuitry for seven segment decoders, display drivers, reference voltage source and a clock. The power dissipation is less than 10mW and the display stability is very high.

The working of this electronic circuit is very simple. The voltage to be measured is converted into a digital equivalent by the ADC inside the IC and then this digital equivalent is decoded to the seven segment format and then displayed. The ADC used in ICL7107 is dual slope type ADC. The process taking place inside our ADC can be stated as follows. For a fixed period of time the voltage to be measured is integrated to obtain a ramp at the output of the integrator. Then a known reference voltage of opposite polarity is applied to the input of the integrator and allowed to ramp until the output of integrator becomes zero. The time taken for the negative slope to reach zero is measured in terms of the IC’s clock cycle and it will be proportional to the voltage under measurement. In simple words, the input voltage is compared to an internal reference voltage and the result is converted in a digital format.

The resistor R2 and C1 are used to set the frequency of IC’s internal clock. Capacitor C2 neutralizes the fluctuations in the internal reference voltage and increases the stability of the display.R4 controls the range of the voltmeter. Right most three displays are connected so that they can display all digits. The left most display is so connected that it can display only “1” and “-“.The pin5(representing the dot) is connected to ground only for the third display and its position needs to be changed when you change the range of the volt meter by altering R4. (R4=1.2K gives 0-20V range, R4=12K gives 0-200V range and R4=0 gives 0-2V range).
Circuit diagram.

Notes.

• Assemble the circuit on a good quality PCB.
• The circuit can be powered from a +/_5V dual supply.
• For calibration, power up the circuit and short the input terminals. Then adjust R6 so that the display reads 0V.
• The ICL7107 is a CMOS device and it is very sensitive to static electricity. So avoid touching the IC pins with your bare hands.
• The seven segment displays must by common anode type.
• I assembled this circuit few years back and it is still working fine.

Description.
Here is a very simple circuit diagram of a frequency to voltage (F to V) converter. Such a circuit finds numerous applications in projects like digital frequency meters, tachometers etc. The circuit is mainly based on a LM555 timer IC. The IC is wired in mono shot mode to convert the input frequency into a fixed pulse width, variable frequency PWM signal. Resistors R4 and capacitor C2 provides the necessary timing for the circuit. The transistor T1 forms a discharge path parallel to C2 which is necessary for re triggering the IC. Capacitor C1 acts as an input DC decoupler.
Circuit diagram.

Notes.

• The circuit can be assembled on a Vero board.
• Use 12V DC for powering the circuit.
• LM555 must be mounted on a holder.
• The output of the circuit is not a pure DC but a PWM waveform. Additional circuitry is required to convert this PWM waveform to pure DC.