Description.

The micro ampere meter shown here is basically a DC millivolt meter.The circuit gives full scale deflection for 0.1V input.The current to be measured is passed through a known resistance R and the voltage drop across it is measured.Here the IC 1 uA 741 op-amp is wired as a non inverting amplifier.

Circuit diagram with Parts list.

Notes.

• Assemble the circuit on a good quality PCB or common board.
• Use a +9V/-9V dual power supply for powering the circuit.
• The IC 1 ,uA 741 must be mounted on an IC base.
• The table given below shows the relationship between different values of R and the current that will give full scale deflection.

Description.

A wide range square wave generator using IC uA741 is shown here.The circuit uses positive feedback for Schmitt trigger action and negative feedback for timing of the wave form.

Let us presume that the output is high and the capacitor C1 is fully  discharged.C1 now starts charging via R2 and R1.When the voltage across C1 rises above that the Junction of R3 & R4,the output quickly switches to fully negative voltage.C1 now starts discharging and charges in the opposite direction.Again,when the negative voltage across C1 falls below that at pin 3,the circuit switches back quickly to the fully positive output value.The cycle repeats endlessly.

The frequency of the square wave can be varied by varying POT R1.The frequency range of the circuit depends on the value of R3,R4 & C1.

Circuit diagram with Parts list.

Notes.

• The circuit has to be powered from a +9/-9 V  DC dual power supply .
• The IC 1 must be mounted on  an IC base.
• The frequency of output can be varied by varying POT R1.
• The frequency range  can be adjusted by changing the value of R3,R4 or C1.
• The table given below gives the relationship between the values of C1 and the frequency range covered.

Description.

The main part of this doorbell circuit are two NE555 timer ICs.When some one presses switch S1 momentarily ,the loud speaker sounds a bell tone as long as the time period of the monostable multivibrator built around IC1.

When the switch S1 pressed, IC1 is triggered at its pin 2 and output pin 3 goes high for a time period previously set by the values of POT R4 and POT R5.When the output ofIC1 goes high it resets IC2 and it starts to oscillate to make a bell sound through the speaker.The IC2 is configured as an astable multivibrator whose oscillation frequency can be varied with the help of POT R5.By adjusting the values of R4 & R5, modifications on the tone are possible.

Circuit diagram with Parts list.

Notes.

• The circuit has to assembled on a good quality PCB or common board.
• The IC1 & IC2 has to be mounted on IC holders.
• Power the circuit from a 9V battery or 9V DC power supply.
• Switch S1 is push button switch.

Description.

An AC coupled unity gain  gain voltage follower operating on a single supply is shown here.The voltage divider network of R1 and R2 provides a DC voltage equal to half the supply voltage to the non-inverting input of the 741.The output DC voltage therefore stands at half the supply voltage.The output signal swings above and below this value.The standing DC voltage at pin 6 does not matter because the output is coupled to the next stage via a capacitor.

The input impedance is equal to the value of R1 and R2 in  parallel, i.e .500K Ohms in this case.Because of hundred percent negative feedback ,the output impedance is very low.For low frequency applications C1 and C2 can be replaced by electrolytic capacitors of large values.

Circuit diagram with Parts list.

Notes. 

• Assemble the circuit diagram on a good quality PCB or common board.
• Power the circuit from a 12V battery or 12V DC power supply.
• The IC 741 has to be mounted on an IC holder.

Description.

Just about any power transistor can be used in  this megaphone.Its suitable for boats,playing fields etc.the transistors Q1  & Q2 are the HEP-230 type which are easily available in the market.The transistors are parallely connected to handle the required power and speaker matching.The microphone is a carbon type like that used in telephone hand sets.If  a regular carbon mike is used the push-to-talk (PTT) switch can be connected in place of S1 to provide PTT operation.There will be no warm up or “capacitor charge time.

Circuit diagram with Parts list.

Notes. 

• The circuit can be powered from a 12V battery or 12V DC power supply.
•  The POT R1 can be adjusted to obtain maximum volume with minimum distortion.
• It will be always better to mount the transistors on heat sink.