Should you use a 555 or a Schmitt trigger?

Article By : Robert Heider

Which should be used, the 555 or the Schmitt trigger? It depends.

“We need a circuit that generates a UV LED light pulse for a couple of seconds over about a 30-second period,” said the manufacturer of a portable aftermarket product with a florescence element. The device was to be bathed in UV light, which would regenerate the florescence when not in use. Next, they showed me a dog-eared 555 spec sheet and recommended that I use it.

My mind wandered to something I heard that the late Bob Pease, analog design guru, said: his favorite circuit to use a 555 is a blank piece of paper. Some even consider the 555 a toy.

Later, I discussed the application with another EE, who told me that he had no problem using a 555. For this application, precision is not required, so I started the design.

The design specification called for low duty cycle, that is, time on divided by total time was to be less than 50%. I recalled this EDN Design Ideas article, which shows how to design a 555 circuit with low duty cycle: Design low-duty-cycle timer circuits. This article assumes the output is “high” for a short time, less than 50%. Its passive component calculations are complicated, however, and it uses an additional diode to shorten the duty cycle.

Wow the engineering world with your unique design: Design Ideas Submission Guide

Another way to accomplish a low duty cycle with the 555 is to connect from the supply, through the load, to the output pin, rather than trying to drive the load. This approach is workable for this design; because the objective is to provide a UV light pulse, the electrical signal levels can be at any level.

For my astable design, I used a capacitance value of 1 microfarad (µF) and a maximum resistance of 10 megohm (Figure 1). This appears to be within the device’s design capabilities, so it is reasonable to expect that the proposed design should work.

555 timer test circuitFigure 1 The 555 timer test circuit is configured for a low duty cycle.

The circuit was built and tested on bread board. It worked well and the customer was pleased, but the academic in me, as well as in tribute to Bob Pease, wanted to submit an alternative design using a Schmitt trigger.

Schmitt trigger

I recalled another Design Idea that used an op amp and an RC network as a Schmitt trigger to create a time delay that helped prevent overheating of an instrument. My application of interest required an astable design, shown in Figure 2.

schematic for Schmitt trigger in astable operationFigure 2 The Schmitt trigger is configured for astable operation.

The RC network, with capacitor voltage at the minus () terminal, together with the bias terms at the plus (+) input of the op amp result in a sawtooth waveform (Figure 3).

The timing can be calculated through natural logarithms. The switch points, Vbh and Vbl, are based on the feedback resistor, R10, and the two series resistances, R1 and R8, at the op amp’s plus (+) terminal. The op-amp output will switch between high at 4.1V to low at 0.7V. Vbh and Vbl are calculated as 3.0 and 1.66V respectively.

sawtooth waveformFigure 3 The sawtooth pattern of the capacitor voltage turns the Schmitt trigger into an oscillator with low duty cycle.

Assume the op-amp output is high and the negative side is at a lower value. In that case, capacitor C1 will charge toward the 4.1V high value through R15 and diode D10. A small current will also flow through R3. Once the capacitor reaches the Vbh value, the op amp will switch and the capacitor will discharge to approaching 0.7V through R3; the diode is now reversed biased.

For the charging part, R = 165k (two 330k in parallel) and C = 15 µf;

Schmitt trigger charging part equation

For the discharging part, R = 2.2Meg

Schmitt trigger discharging part equation

Schmitt trigger breadboard circuitFigure 4 The breadboarded Schmitt trigger worked as expected.

This circuit too was breadboarded and tested (Figure 4). Both designs were presented to the group, and both worked well. Both used a 5-V supply, which can be supplied through a USB connector. It is interesting to also note the similarities of both designs: a 555 uses a set-reset flip-flop and two comparators that may be a Schmitt trigger.

/wp-content/uploads/sites/3/2020/06/contenteetimes-images-edn-design-ideas-di-button.pngSo, which was to be used, the 555 or the Schmitt trigger? I reminded the group that just having two designs work on breadboards is a long way from realizing a commercial product. Another recommendation was that they show the designs to companies that specialize in taking development to a commercial product.

I further suggested they consider component reliability. Their final design was to have the circuit and battery supply housed in a portable unit that would have to function in a wide range of environments. So, the actual selection should be based on environmental and reliability specifications.

Robert Heider is a retired engineer with over 50 years’ experience with emphasis on the design of advanced process controls and process development.

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