# 555 as a Schmitt Trigger and an ON delay timer?

I've scoured the internet trying to find this out to no avail. I would like to use a 555 timer to produce a Schmitt trigger with an ON delay of 20-30 seconds. Is this possible? Or will I have to use two timers? If I can do it with just the one 555, can someone point me in the right direction as to how? Seems like you should be able to, but danged if I can figure it out!

• Do you want to make monostable multivibrator that has schmitt trigger input? What is its hysteresis needed for? Picture of desired waveforms would be most helpful. – venny Sep 9 '14 at 18:43
• I don't think you can implement hysteresis per se using the 555 because its signal input is a window comparator which is already hysteretic. Could it be that you're just looking for a 20 to 30 second ON delay after the input drops below a certain level? If so, what does your input signal look like and what do you want the circuit to do when the input signal reverts to its OFF state? If not, can you explain in a little more detail what it is you want to do? – EM Fields Sep 9 '14 at 23:47
• @JohnSmith I've included a timing diagram at the beginning of my answer, indicating how I interpreted what you wanted; if it is wrong, please add a diagram to your question – tcrosley Sep 11 '14 at 12:06

The description was a little ambiguous; this is how I interpreted it:

After the input reaches a certain threshold level (dotted line), the output goes high for 25 seconds.

The functions of a Schmitt trigger and a monostable need to be handled by two separate 555s. Luckily, you can get two 555s inside one 14-pin IC called a 556. Here is a circuit which uses the first half of the 556 as a Schmitt trigger, and the second half of the 556 as a 25-second timer. If you want a different value for the timer, change R3.

Note because of the typical tolerance of electrolytic capacitors, ±20%, the accuracy of the timer will be about the same assuming you use a 1% resistor for R3. If you want tighter timing, you can get a 10% tantalum capacitor, but they are relatively expensive (\$5). They do make 5% tantalum capacitors, but only in surface mount packages.

When the input voltage is above 2/3 Vcc, the output of the Schmitt trigger goes low, triggering the monostable. When the input is less than 1/3 Vcc, the output of the Schmitt trigger goes high. Note that the TLC556, linked to earlier, has a Vcc range from 2v to 15v which gives a lot of flexibility in setting the hysteresis levels.

There are variations on this circuit, including adding a capacitor to the input and a voltage divider to change the trigger voltage. This article has lots more information.

• People, please! It's spelled "Schmitt trigger". – Rennex Sep 9 '14 at 19:43
• I don't think your circuit will work, since what he's asking for is some voltage to trigger a 20 to 30 second delay after which an output will go hot and, ostensibly, stay there until the input voltage goes back to whatever state is necessary to turn the output OFF. There's also another issue in that for the 555 to output a pulse, the TRIGGER input must go high before the output pulse is supposed to end, otherwise the output will just follow the trigger input, but inverted. – EM Fields Sep 10 '14 at 0:24
• @EMFields No, I disagree, when the input goes above the trigger level of the Schmitt trigger, the output OUT1 will go low, triggering the monostable, whose output OUT2 will immediately go high for the 25 seconds. The OP never stated the input could go high for the entire 25 seconds, I would doubt that's the case. Anyway, that could be fixed by adding a capacitor on the input as I discussed in my answer. – tcrosley Sep 10 '14 at 0:34
• OK, but an ON delay generally describes a box with a switch on it that causes something to change state - in this case - 20 seconds after the switch is thrown and then to immediately revert to its former state when the switch is returned to its former state. – EM Fields Sep 10 '14 at 0:45
• @EMFields I took it to mean the output is high during the delay period. The OP was not really clear about how things were to work. If I misinterpreted it, the circuit can easily be changed. I was mainly trying to make the point that you could do this in one IC (556) instead of several 555s. – tcrosley Sep 10 '14 at 3:11

A 555 timer is composed of comparators that operate over modulation of pulses. The three modes: astable, bistable, and monostable are the operating modes you can set the 555 timer to. Apparently a poroperty of a Shmidt Trigger is ,hysteresis, thus creating dependencies based on outputs from previous modulations. Therefore, it would take multiple timers to output a customized or variable output. You might be able to chain 555 timers to create a model similar too the Schmidt Trigger but I feel as though the 555 timer works on an analog wavelength where as the Schmidt trigger works over a digital wavelength.

• A Schmitt trigger can certainly be made from a comparator. And digital is a polite fiction. Everything is analog. – Carl Raymond Sep 9 '14 at 21:56
• The charge on an electron isn't discrete? – EM Fields Sep 10 '14 at 0:27