555 Circuits and Ideas

A compilation of 555 related circuits in my other blogs and pages. User Interface circuits, Long Duration Timer Circuits  and Infra Red LED Flashers.

Power supply with battery backup - The 555 Astable is used to generate a AC signal from which a negative voltage is generated, A 79L05 which is a low power TO92 equivalent of 7905 a negative -5 volts regulator is used as -5 volts load is less. A TO220 7805 is used for the +5V supply....



Audio Visual User Interface with Ack - Every Hour “+V UR” Goes High for 120 Seconds. The Buzzer Sounds and a Red LED turns on. The Guard has to respond by Pushing the Switch. The Green Light Flashes and the external Flip-Flop logic brings “+V UR” Low, The Buzzer Sound Stops and Red Light Goes off....





Display On Timer with 555 -  When the “Display Now” is pressed it triggers the Monostable Multivibrator made of a 555. The output “EN” goes high for the time duration defined by C27 and R71. The 555 output as you know can drive more than 200mA for quite some time without much heating up. Many LEDs can be driven with the current limit resistors. I used CMOS chips to drive the LEDs, this circuit was used for the Logic only.



Frequency Divider 74HCT4040 - U1 7555 is a CMOS version of 555. The 555 here is in Astable Oscillator mode, C1 and C4 are decoupling capacitors 0.1uF value, ceramic disc. The output is around 100kHz, If C3 is plastic or mica the frequency output will be stable with temperature. It is better to use a crystal oscillator.





InfraRed LED Flasher for Optical Switch - 555 is used as an astable oscillator and it flashes the Infra red LED D1 at a high speed, The object close to this LED reflects the light along with the ambient light which may also be sunlight.

Initial Accuracy of 555

The repeatability of the timing over usage in days, months or even years. Change in performance from device to device for the same RC values and Supply voltage. NE555 the value is 1% Initial accuracy, that is excellent. Use a Plastic Capacitor for C and Metal Film Resistor for R for best results.

Timing Drift with Temperature

In monostable mode the 555 has a 50 ppm/deg C and in Astable it is around 150 ppm/deg C. ppm is parts per million. How to understand : 1% = 1/100 and 1ppm = 1/1000,000 ... So 1000 ppm = 0.1% and 100 ppm = 0.01%. In other words a 10 deg C change in temperature may cause a drift of 0.1% in timing of Astable.

Timing Drift with Supply Voltage

Typical drift is 0.1% per volt. That means the timing will change around 0.1% if you change the supply voltage by one volt. If at 5V the timing is 100mS, at 8V the timing will change by 0.1% x 3V = 0.3%. Now see the datasheet and graphs for the impact of this marginal drift. Use a Regulated power supply, keep RC far from hot parts, use Plastic caps and Metal Film Resistors. Remember even copper tracks conduct heat.

Images above are from the Original Signetics 555 Datasheet

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