A pulse generator is a circuit that generates pulse of certain duration when a switching signal is applied to the circuit. The switching signal can come from like tactile momentary switch, on/off pulse from microcontroller or boards like Arduino. So a small input pulse generates a output pulse whose duration can be controlled.
A simple pulse generator can be designed with controller for input and output to an RC circuit. As it is well known that time constant of RC network can be fixed with suitable value of R and C. We need a input circuit that can charge the capacitor C and output circuit that can discharge the same capacitor. We can use BJT transistors at the input and the output to control the input switching signal and for discharging the RC network in response to the input switching signal.
The following circuit diagram one way of implementing pulse generator using transistors.
In the circuit diagram, RC network is situated between the transistor Q2 and Q1. All transistors are 2N3904 BJT transistors. Basically, the transistors Q1 and Q4 forms the input circuit that gets input pulse which will cause the RC network to discharge through the transistor Q2. The transistor Q2 is the output transistor which will discharge capacitor C in response to the input voltage. The transistor Q4 is to invert the input signal. When the tactile switch is pressed, the base is grounded or low and the collector is saturated or high at +5V. This high going input pulse will trigger the transistor Q1 which in turn discharges the capacitor C through Q2. The transistor Q3 is used to maintain the collector of Q1 at ground while the capacitor is being discharge at the output regardless of the input pulse. Otherwise, the output pulse width will be same as the input pulse width. That is without Q3 transistor, the output pulse follows the input pulse.
The R and C values used are 47KOhm and 47uF each which gives a pulse width of 2.2 seconds. You can use the online RC time constant calculator to find out the R and C values for give time period. The following shows the graph of the input voltage(+5V pulse) and the output pulse generated.
The output pulse voltage level is low compared to the input voltage. Also the output pulse current is also very low. To drive loads such as motors, LEDs, light bulb, buzzer etc both voltage and current amplification is necessary. BJT amplifiers or MOSFET amplifiers can be used which can designed easily using the online BJT amplifier design calculator or mosfet amplifier design calculator.
As an example of application of this pulse generator circuit, the following video demonstrates how the pulse generator can be used to drive a buzzer. When the momentary switch is pressed, the buzzer starts sounding for 2.2 seconds.