Alarm circuit triggered by fluorescent tubes

[Mike G]

Hello all. I hope you can help me solve this annoying problem.

I have built a simple burglar alarm for my caravan. It works off the 12V
caravan's system which is supplied by its own leasure battery. The circuit
uses a 555 timer. The normally closed sensors are used to by pass (or
short??) a resistor. Therefore if a sensor contacts open, the resistor
creates a voltage dip on the trigger pin of the 555 timer, thereby
activating the alarm in monostable mode. Hope all that is clear???

Anyway, here is the problem. When at home, I plug the caravan into the mains
from the garage. This charges the battery in the caravan, and also supplies
a mains voltage socket, in the caravan. However, if I switch on the lights
in the garage (fluorescent tubes), whilst the caravan alarm is set and its
connected to the mains, it triggers the alarm. I am guessing, that the tubes
are somehow sending a spike or something into the caravans electrics. I
tried inserting a 22 microfarad capacitor across the supply of the alarms
circuit, but it had no effect. I'm new to electronics so any suggestions how
to prevent this would be gratefully received!!

Many thanks in advance.
Mike.

 

[Sir Charles W. Shults III]

Do you have a bypass capacitor on pin 5 of the 555 chip?

Cheers!

Chip Shults
My robotics, space and CGI web page - http://home.cfl.rr.com/aichip

 

[Mike G]

"Sir Charles W. Shults III" <aichipREM@OVEcfl.THISrr.com> wrote in message
news:Aym2b.86283$qg3.4461013@twister.tampabay.rr.com...

Do you have a bypass capacitor on pin 5 of the 555 chip?

No! Pin 5 is not connected to anything! What value capacitor should I use,
does it matter if its electrolytic, or not??

Many thanks for you reply.

Mike.

 

[Mike G]

Many thanks for the explanation Jon. Cheers.

Mike.



"Jon" <gremlin@fastmail.fm> wrote in message
news:3f52959f@news.alphalink.com.au...

Hers's some info that will make it easier to understand the problem.

http://www.uoguelph.ca/~antoon/gadgets/555/555.html Got it from this
link. Let me know if this fixes it!

Pin 5 (Control Voltage): This pin allows direct access to the 2/3 V+
voltage-divider point, the reference level for the upper comparator. It
also allows indirect access to the lower comparator, as there is a 2:1
divider (R8 - R9) from this point to the lower-comparator reference
input, Q13. Use of this terminal is the option of the user, but it does
allow extreme flexibility by permitting modification of the timing
period, resetting of the comparator, etc. When the 555 timer is used in
a voltage-controlled mode, its voltage-controlled operation ranges from
about 1 volt less than V+ down to within 2 volts of ground (although
this is not guaranteed). Voltages can be safely applied outside these
limits, but they should be confined within the limits of V+ and ground
for reliability. By applying a voltage to this pin, it is possible to
vary the timing of the device independently of the RC network. The
control voltage may be varied from 45 to 90% of the Vcc in the
monostable mode, making it possible to control the width of the output
pulse independently of RC. When it is used in the astable mode, the
control voltage can be varied from 1.7V to the full Vcc. Varying the
voltage in the astable mode will produce a frequency modulated (FM)
output.

In the event the control-voltage pin is not used, it is recommended
that it be bypassed, to ground, with a capacitor of about 0.01uF (10nF)
for immunity to noise, since it is a comparator input. This fact is not
obvious in many 555 circuits since I have seen many circuits with
'no-pin-5' connected to anything, but this is the proper procedure. The
small ceramic cap may eliminate false triggering.

Mike G wrote:
"Sir Charles W. Shults III" <aichipREM@OVEcfl.THISrr.com> wrote in
message
news:Aym2b.86283$qg3.4461013@twister.tampabay.rr.com...

Do you have a bypass capacitor on pin 5 of the 555 chip?




No! Pin 5 is not connected to anything! What value capacitor should I
use,
does it matter if its electrolytic, or not??

Many thanks for you reply.

Mike.