Pulse Generator Circuit Cum Timer Circuit

A pulse generator circuit cum timer circuit is a multipurpose instrument. In the case of the electronics field, it is that apparatus without which the design of the project becomes a complicated task. Multiple types of such circuits might have been available in the market. But the circuit we are going to discuss here overshadows them with the range of features it offers variable frequency, variable pulse width, and variable amplitude. To add more to it, we can use this instrument like a timer too in case of need. It can also be included in direct relay operation since it offers high voltage (up to 24V).

The pulse generator circuit grants a wide frequency response of 0.001 Hz to 5 MHz, and a wide variation of pulse width: 100 nanoseconds to 10 seconds. And, the main point is it is reasonably accurate. Similarly, it also gives true positive/negative pulses (+24V or -24V) and has a separate output arrangement for TTL gates (4.35V). These outputs thus arranged are so arranged that they are protected from short circuit conditions.

Circuit Description of Pulse Generator Circuit Cum Timer Circuit

The major component of the project ‘Pulse Generator circuit cum Timer circuit’ is 7404 TTL hex inverter IC which is used as an astable multivibrator. This IC can be used to get up to 18 MHz but it is operated to give up to 5 MHz only.

Components like R₁, C_1, and VC₁ are used to set the frequency of the generator. Among them, R₁ and C₁ are fixed and VC₁ is an air dielectric gang condenser whose value is variable up to 500 pF. This condenser is provided with two separate terminals and one common terminal. Out of which the two separate terminals are shorted. The terminals are shorted and the other common terminal is connected across C₁. And, by adjusting or rotating gang condenser VC₁, we can obtain the output frequency varying from 5 MHz to 1 MHz as required.

The output thus obtained is supplied to multiple ICs to produce a wide range of frequencies. Firstly, it is fed to IC₂, a 7490 TTI frequency divider which is functioning as a divider-by-five in the circuit. This is then followed by IC₃ which is another 7490, used as a divider-by-two. The output of IC₃ is further divided by C₄ acting as a divider-by-five. This alternate division by two and by five is repeated by IC₅ through IC₁₁. All these ICs are connected in cascade and to a band-switch like IC₃ and IC₄.

This arrangement thus designed ensures the selection of a wide range among 5MHz to 1 MHz, 1 MHz to 500kHz, 500kHz to 100kHz, 100kHz to 50kHz, 50kHz to 10kHz, 10kHz to 5kHz, 5 kHz to 1 kHz, 1 kHz to 500Hz, 500Hz to 100Hz and 100Hz to 50Hz through band-switch SW₁ as shown in the figure. In the same way, band-switch SW ­₂ enables the selection of a low-frequency range from 50Hz to 10Hz, 10Hz to 5Hz, 5 Hz to 1Hz, 1Hz to 0.5Hz, 0.5Hz to 0.1Hz, 0.1Hz to 0.05Hz, 0.05Hz to 0.01Hz, 0.01Hz to 0.005Hz and 0.005Hz to 0.001 Hz which is followed by “off” position.

This arrangement is then followed by IC₁₂ which also operates as a monostable multivibrator. It is a 74121 TTL IC that can control the width of a pulse from 40 nano-sec to 28 sec. The range selection switch selects a timing condenser among C₂ to C₁₁ and a continuous variable is obtained through VR₁.

To get a fixed output voltage(4.3V) at the TTL output, the output of monostable IC₁₂ is given through switch SW_4A to high-speed switching transistor T₁ (a 2N2369A). Resistor R₄ functions as a limiting resistor to protect the transistor from short circuits. Resistors R₈ and R₁₁ also perform similar functions. The output amplitude can be kept at either 12V or 24V. The only changes that have to be made are:

The power supply circuit is simple. IC₁₃ is a 7805 (three-pin 5V regulator IC) and the positive supply is simple Zener regulated. IC₁₃ must be mounted on a large heat sink. It is better to have this IC in the T03 pack for good heat dissipation as under the worst condition the current will be 50*15, i.e. 750 mA.  

Construction of Pulse Generator Circuit Cum Timer Circuit

One must take extra care in the construction of this circuit since it is a TTL high-frequency circuit. Getting started with, firstly connect IC₁₃ and other power supply components and make sure you are getting +5V supply concerning the earth. Thereafter, connect IC₁, C_1, and R₁ keeping VC₁ unconnected, and then observe the waveform.

Once you obtain a waveform about (3V) then connect VC₁ and examine the waveform again. You should get the frequency range of 5MHz to 1MHz. The range may not be precisely the same as stated, however, you can obtain the output around this range or even a slightly wider range of frequency by the full rotation of the gang condenser.

In case you do not get the full range, check on IC₁. If that doesn’t work, check C₁ and R₁, and lastly check VC₁. Sometimes, it can be due to the dust particles contained in the air gang condenser or even the plates might be shorting. In that case, clean the dust or carefully separate the plates. And then it should start working. During cleansing, don’t hesitate to clean this gang condenser as well so that you may not face the same difficulty afterward. Now connect IC₂ and check the output. Then connect IC₃ and check the output again, and repeat the same up to IC₁₁. Do not connect switches SW₁ and SW₂ yet. Now connect IC₁₂ with SW₃, C₂ to C₁₁, R_2, and VR₁. Keep the input wire, i.e. the wire connected to pins 3 and 4 of IC₁₂ to 10KHz, and check on each range of SW₃ to ensure that you are getting proper output.

As mentioned in earlier paragraphs, this is a high-frequency circuit, and therefore all wires connected to SW₃ (as also to SW₁ and SW₂) must remain parallel to each other. You can make use of insulation tape to achieve a parallel arrangement. On the other hand, the connections to VR₁ and VC₁ should be done through a twin lead, like the wire used for a TV antenna.

When you are sure the circuit is operating properly, connect transistors T₁ and T₂ and the remaining components of the circuit i.e. resistors. The circuit has been divided into two sections only to avoid complexities in case of error prediction in the circuit which is obvious in such projects. Hence do not proceed to the next portion unless you are sure that the preceding circuit functions as required.

Calibration of Pulse Generator Circuit Cum Timer Circuit

For the effective outcome of the project, calibration must be done with a good oscilloscope that has a frequency response of at least up to 10 MHz. Keep SW₁ in the first position and SW₃ off. Then rotate VC₁ and mark on 5MHz, 4MHz, 3MHz, 2MHz, and 1MHz positions. During rotation, one should note that the full rotation of the gang condenser might vary the frequency from 5.8 MHz to 0.6 MHz but do not calibrate the remaining frequencies since these are not useful frequencies in the project we are discussing.

Now keep SW₁ in the second position and check if the previous markings coincide with the positions of the gang condenser for 1 MHz, 0.9 MHz, 0.8 MHz, 0.7 MHz, 0.6 MHz, and 0.5 MHz. Following the same pattern, switch SW₁ to the other positions to verify the calibration further.

Similarly to verify the calibration of SW₂, use LED in series with 220-ohm, 1/4W resistor and connect it to TTL positive output and note the precise time between LED flickering. To calibrate the pulse width, set frequency 5MHz, keep SW₃ in the first position, change VR₁ and take note of divisions like 50n, 60n, 70n, 80n, 100n sec, etc. Now without changing the set frequency, calibrate each range. Lastly, calibrate the amplitude.

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Some useful hints | Pulse Generator Circuit Cum Timer Circuit

1. The gang condenser must be kept in a dust-free plastic box. For better system stability make the full chassis dust-free if possible.
2. Use shielded wire for output.
3. To drive a relay at a low frequency, use the adapter given in fig.4. It can be kept inside the cabinet, with the rest of the circuit, keeping two output terminals for the relay.

PARTS LIST OF PULSE GENERATOR CIRCUIT CUM TIMER CIRCUIT