Schematics of delabs

This is related to the earlier post. This was done to save battery power. The output of some equipment may be in the form of LED Annunciators In the Security Guard Monitor, an Array of 24 LEDs would show the Status of alertness along with the day, this was scrollable.


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.

Hobby Projects - LED Circuits and Small Test Tools.

This circuit was a part of the user interface like the previous one. You could use it for voltmeters or even backlight for LCD.

Example - When you press the switch the Time of a clock can be seen in darkness for a few seconds.

This circuit gives a burst of pulses to fire 2 SCRs, when pin 4 is taken to 12V the SCR is turned on, to use this circuit you need 12V short pulses phase shifted with respect to AC sine wave on bridge, like this you can control the bridge from near 0% to near 100% ON.

Drive or Fire Thyristors SCR with Pulses



Learn about Thyristors here Power Electronics

That way battery banks can be charged, electroplating can be done, current and voltage can be controlled with opamps, thyristors are very rugged compared to transistors and MOSFETS in that order.

This shows how to OR gate two 555, when one 555 cycles at a low frequency a valve turns on an off, the second 555 stretches the ON duration of the pulse with a diode OR gate.



The OR output uses sample and hold to get the stable analog data from a sensor after the actuator has gone OFF, this ensures correct reading.

OR gate with two 555

555 is a fundamental Mixed Signal Circuit as it can be made into a VCO using Pin-5. If you see old exar databooks, you can see 555 and PLL and Tone decoders all applications compiled in one base. I feel the Venerable Signetics 555 "Architecture" and Intersil ICL8038 'CMOS' were inspiration behind early communication chip designs, Moving from Bakelite Telephones to Compact Push Button Electronic Phones and more.

When the microcontroller hangs due to a spike, EMI or RFI etc. the 7555 will reset the uC, if proper power supply design is done above circuit can also give a clean power on reset, the above circuit you should modify to suit your design.

80C39-8749 MCS-48 Examples and code

Better still use a uC with watchdog built in like some atmel chips, or use the watchdog chips from maxim which can also do RAM battery management.



The circuit was developed over a old TI application note, 7555 i think fairchild may be making it, 7555 is CMOS version of 555 timer, advantage of 7555 is that it can go to higher frequency, low power consumption, the disadvantage is its output drive mA is not as good as 555. now why i put it here was that you can see how charge and discharge paths are separated with diodes.

555 watchdog for uC and uP systems

Watchdog Timers can reset a microontroller or microprocessor system when it goes into an endless loop or gets stuck. 555 is used here to keep a tab on the uC activity. It resets when it senses the uC is not pinging it anymore.

This circuit is based on a very old application note from exar, in this the frequency is fixed by IC1 and IC2 -P1 controls the duty cycle. you need to compute the R and C values to get what you need,



You have to study the circuit and do something more innovative perhaps, just copying is ok for learning but it will get you nowhere, so learn and then innovate,

Fixed frequency var DC 555

You can even make a PCB at home to learn, but it is always good to get PCBs done by a PCB vendor, but you should understand his problems, then you will design well, so make a few PCBs.