Alarm circuit question - Page 1
Forum section: electronics / beginners

author: DDT
author_rank: Contributor
timestamp: April 06, 2016, 11:30:40 pm
content: Hi, I've come across this interesting alarm circuit, however as I am just a beginner, I need your help to understand some part of it. Referring to the picture, the circuit comprises of a motion sensor switch (S2), buzzer (24), RC delay and SCR. When switch S1 is activated to NO position, the capacitor 14 starts to charge through resistor 13 and at this time SCR can not conduct because the gate of SCR is grounded via resistor 26, thus the device remains inactive during this interval of time. The motion sensor during this time CAN BE either open or close, it doesn't matter while the capacitor is charging, it won't activate the buzzer. However, after the capacitor is fully charged any movement of motion sensor, either from close to open or vice versa triggers the SCR.My questions; How exactly the SCR is triggered via both modes of sensor operation and what is the role of capacitor 34 and resistor 32?P.S. the motion sensor is type of ball in the container, while the capacitor is charging the ball could be in contact with metallic container(close) or not ( open).

author: ElektronikLabor
author_rank: Regular Contributor
timestamp: April 07, 2016, 05:28:24 pm
content: There aren't any values marked on the components, so it's hard to say how it's behave.Here my guess:The C14 ensures, that the alarm stays on for a while after it was triggered. R13 is to big to power the buzzer, hence the buzzer turns off when C14 is uncharged. The Value of C14 determines how long the buzzer stays on after it was triggered. I presume it's a big electrolytic Capacitor.C34 delivers a current spike to the base of the SCR after triggering but when it's charged, the current is limited by R32. I presume the reason for the C34 is, that even short pulses of S2 ensures, that the alarm is properly triggered. But I'm not sure about it

author: ade
author_rank: Unknown
timestamp: April 07, 2016, 05:38:38 pm
content: You can look up the patent for that alarm circuit here:https://www.google.com/patents/US3706867See the section "DESCRIPTION OF THE PREFERRED EMBODIMENT". It is written in legalese but has a good description of the intended operation of the circuit including the roles of capacitor 34 and resistor 32.

author: DDT
author_rank: Contributor
timestamp: April 07, 2016, 07:14:19 pm
content: Hi guys, thanks for the replies. I have attached the modified version of the circuit which shows the tilt ball. I think the mechanism is as follows; please correct me if I am wrong.1. Arming: The S1 switch closes and C14 starts to charge. During the charging, the resistor 26 keeps the gate of SCR grounded so the interruption of tilt switch won't activate the device.2. After the timing sequence is finished and C14 is fully charged, assuming the device is stationary, there are two possibilities:a) The tilt switch is closed- this provides a path for C34 to be charged. Any movement of the tilt switch means the momentary opening of tilt switch. Will thisinterruption produces a positive voltage at the point 31 so the SCR is triggered?b) The tilt switch is opened- the movement of tilt ball closes the S2 switch and as a result C34, and R32 start to draw current. This leads to build up of positivevoltage at point 31 and fires the SCR.Thank you in advance.

author: bobcat
author_rank: Regular Contributor
timestamp: April 10, 2016, 10:48:57 pm
content: C24/R26 provide a time delay when S2 is closed before the SCR is triggered. This prevents false triggering in case of bumping etc. R32 discharges C24 when S2 is open again.

author: DDT
author_rank: Contributor
timestamp: April 11, 2016, 12:06:17 am
content: Quote from: bobcat on April 10, 2016, 10:48:57 pmThanks bobcat you are correct, however, the question is after the time delay, how change of state of the switch S2, either from open to close or vice versa triggers the SCR? the circuit looks simple but hard to understand and the patent doesn't help much. Thanks