555 watchdog for uC and uP systems - del00013
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.
Digital to Analog Converter with uC Watchdog
This is another way i did the same thing.
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.
Another way i have used a 555 -
High Resistance Meter
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.
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.
See another Application of 555 in a product i once
Microohm Meter with LED Analog Bar
It maybe a good idea to keep the Watchdog timer
outside main uC silicon chip. The Watchdog should
not be an intelligent circuit. It has to be a
generic mixed design so it operates reliably in
See Larger Circuit. 555
Watchdog. Edit the circuit source file del00013.zip.
555 Pin 3 Inductive Latch up
Inductive Load Latch-up A Negative voltage at pin 3
can cause a latch-up. The Solution is to add
two diodes as shown. Parallel to Load draws away the
One more method is to have a Freewheeling Diode
across the Solenoid or Relay-Coil that the 555 is
driving. Driving Lamps and LED is not a problem with
555. Driving coils are best done with an external
NPN transistor. You can also use the Astable
configuration to pulse drive LEDs without a large
Resistor. Use a 30% Duty Cycle and High Clock Rate.
Sensible User Interface is as old as 555, see an
early application note of a touch switch, useful for
embedded systems. Use a 7555 here, cmos for more
sensitivity. The Finger Triggers the 555 at pin 2.
The Oneshot will go high for RC period. Use a
Flip-Flop as latch or memory.