Design Ideas: January 4, 1996

The transition counter of Figure 1 is an easy way to determine if the temperature in a heater is stable. The stable state maximizes the transitions. This circuit works like a digital filter with controlled frequencies and is simpler than many other analog- and digital-circuit alternatives.

The figure shows a functional diagram of a commercial switch control that operates over a particular range. The LED array indicates the thermal stability. The 555 timer, which operates as an astable oscillator, generates the clocking signal for each sample. In this case, the timer output’s high period controls the counter time, and, by adjusting R₁, you can adjust the number of LEDs that switch on. The equations for the timer are as follows:

In this design, R₁ is greater than or equal to 10ŚR₂. (DI# 1809)