EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: mazza85 on July 14, 2012, 12:39:16 pm
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I've made a very simple garage door alarm using this circuit http://www.kpsec.freeuk.com/projects/lalarm.htm (http://www.kpsec.freeuk.com/projects/lalarm.htm), modified by eliminating the 100k trimmer, 22uF electrolitic to replace the 10uF for longer bleep duration and using a magnetic reed switch instead of the LDR. THe output drives a relays via a bc547 transistor.
When the circuit is powered by a 12V battery pack everything is fine (especially the power-on reset).
If I power the circuit with a switching wall wart the power-on reset don't work (the output is high at the start).
How ca I solve this? Or I have to use a linear power supply (LM7812 p.e.) ?
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Make the 0.01uF capacitor 0.22uF, to give a longer reset time. Most likely the power suply is slow to rise in voltage. Add an additional 100uF 16V capacitor across the power pins of the 555 as well, pin 1 and 8. This should solve it.
What is the voltage output of the wall wart?
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Have you measured the output voltage from the wall wart?
Some cheap wall warts have no voltage regulation (yes even if they're switch mode) or it may have a minimum load current requirement.
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Thanks for the answers.
Made the modification as suggested by SeanB ( new schematic attached) but the circuit still sometimes triggers at the power-on
I've measured the output voltage: unconnected 10,66 V / connected to the circuit 10,48 V
I've noticed also that if I touch with a screwdriver or a lead from the dmm either the Vcc, the GND or the base of the BC548 the circuit triggers
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That is mains hash from the switching power supply triggering it then. Change the 1M resistors on pin2 and on the transistor to a lower value, around 10k will do. You might need to use a linear power supply, or connect the negative lead of the supply to an earthed object, or to a earth spike just outside the door.
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That is mains hash from the switching power supply triggering it then. Change the 1M resistors on pin2 and on the transistor to a lower value, around 10k will do. You might need to use a linear power supply, or connect the negative lead of the supply to an earthed object, or to a earth spike just outside the door.
]Good point.
It also may be the mains being coupled across the Y capacitors in the SMPS. Another option would be to connect both power supply rails to earth/ground with 100nF capacitors, if complete earth bonding is not desirable for some reason.