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!

Best Answer

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

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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.

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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.