Circuit produces variable numbers of burst pulses

The add-on circuit in Figure 1 can produce one to 15 burst pulses with the same number of spaces between the bursts at a pulse width (frequency) that an external square-wave generator at the input sets. The add-on circuit produces a variable number of bursts and a variable number of spaces between the bursts by using an external square-wave generator as a source. The project in this design required a TTL burst signal, but resources did not allow for the expense of a burst generator. The circuit basically comprises two hexadecimal, divide-by-16 counters set up so that the counter on the left produces a user-selectable zero to 15 pulses and the counter on the right produces a user-selectable zero to 15 spaces. The two hexadecimal thumbwheel switches select the number of pulses and spaces.




Figure 1: This circuit produces a variable number of burst pulses and spaces.

Counter IC1 controls the number of bursts, and counter IC2 controls the number of spaces. The two hexadecimal thumbwheel switches, S1 and S2, select the count value. Each switch position is numbered zero to 15. S1 controls the number of burst pulses, zero to 15, and S2 controls the number of spaces, zero to 15. For either the IC1 or the IC2 counter to count, Pin 7 must be high. If Pin 7 is low, then the counter remains disabled. For a counter to be loaded with a desired count, Pin 9 must be low and then high. The carry output at Pin 15 is normally low until the counter reaches a count of 15, and then it goes high. When the circuit is powered on, resistor R1 and capacitor C1 form an RC-time-constant power-on-reset circuit at Pin 1. This feature initializes the counters to the zero state upon power-up. After that, the thumbwheel switches set the count value.






When a clock signal arrives at the counters' Pin 2 with the desired frequency, counter IC1 starts counting up, and counter IC2 remains in the off-state because the low signal at IC1's Pin 15 carry output applied to IC2's Pin 7 disables counter IC2. When IC1's count reaches the end (15), it goes high and enables IC2 to count. IC1's carry output also goes through inverter gate IC3A and then to the OR gate IC4's Pin 1. The low signal on one input of IC4-and the fact that, because IC2 is now counting, its carry output at Pin 15 is also low at IC4's Pin 2-means that a low signal appears at IC1's Pin 7, and thus IC1 now becomes disabled. Both IC1 and IC2 counters' Enable pins are cross-connected, so that when one counter is counting, the other counter becomes disabled. The two counters work in this way, back and forth, counting up to 15 and enabling and disabling each other. And finally for the two counters, when the carry output on IC2 goes high, the circuit then, after it reaches a count of 15 through the inverter IC3B, loads a new count or reloads the old count into the counters as set by the thumbwheel switches for the next count.

When IC1 is counting, the output of IC3A (the gate signal), assumes a high level at AND gate IC5's Pin 2. This state allows the clock signal to pass through IC5 unimpeded to the output. The output of IC5 is the burst output. When IC1 is disabled and IC2 is counting, the gate signal from IC3A asserts a low signal at IC5's Pin 2. The output is also low and produces no bursts. You can configure this circuit to produce even more pulses or spaces by simply cascading more counter chips where needed. Also, you can replace switches S1 and S2 by an 8 bit write-output register, making the pulse and space counts software-controlled, or you could apply the gate signal to the control input of a CMOS switch to burst analog signals, such as sine waves at its input.