Square pulse generators
Assignment:
1. Make square pulse generator with Logical circuit 7414
2. Make square pulse generator with Logical circuit 7404
3. Make square pulse generator with timer 555
Generators with TTL circuits and RC element
Generators with RC elements are the simplest of the generators. Their biggest disadvantage
is that they have very unstable frequency and they are not very reliable. The simplest circuit
diagram is in figure 1.
Fig. 1. Square pulse generators with TTL circuits (Schmitt trigger 7414) and RC element
Frequency of oscillations is given by time constant of RC element T=R*C. For bigger values of
R and C we can obtain Lower frequency and for smaller values we obtain higher frequencies.
For R=1kΩ and for C=100nF, the output frequency is approximately 36 kHz
Another circuit diagram for square pulse generator is shown in figure 2. It uses 2 TTL circuits
NOT.
1
R
C
C
1
Output
R
Fig. 2. Square pulse generators with TTL circuits (7404) and RC elements
1
Generator with 555(Timer)
In figure 3, you can see circuit diagram of 555 which is connected like generator.
Fig. 3: Circuit diagram of 555
Fig. 4: Output waveform of 555
Signal period (T) is a time for whole cycle. Frequency is inverse value of period F=1/T.
You can use following equations to obtain the time period and frequency for the generator
with 555 circuit.
T  0,7  C1  (R1  2R2 )
1,43
f
C1  (R1  2  R2 )
T = Time period[s]
f = Frequency [Hz]
R1 = Resistance [Ohm]
R2 = Resistance [Ohm]
C1 = Capacity [F]
Time period can be divided into two parts – time for Low level (Ts) and time for High level
(Tm).
T  Tm  Ts
Time for High level of the output:
Tm  0,7  C1  (R1  R2 )
Time for Low level at the output:
Ts  0,7  C1  R2
Values of R2 and R1 must be in range 1kΩ to 1MΩ.
2
You can use following table to obtain values for components.
C1
R2 = 10k R2 = 100k R2 = 1M
R1 = 1k R1 = 10k
R1 = 100k
1nF
68kHz
6,8kHz
680Hz
10nF
6,8kHz
680Hz
68Hz
100nF 680Hz
68Hz
6,8Hz
1uF
68Hz
6,8Hz
0,68Hz
10uF
6,8Hz
0,68Hz
0,068Hz
(41 per minute) (4 per minute)
Tab. 1: Component’s values
3