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DVR IR Motion Trigger 2012-08-29

A client needed a timer to keep his DVR enabled and recording when the IR motion detector output a pulse. A timer was required to maintain a low signal level for a precribed amount of time upon detection of motion.

However, it was determined after some experimentation that the DVR ignored singular outputs, and thus required a "wake up" pulse, which would not be present when using an ordinary timer configuration. The included circuit allows the client to set the time to record after motion detection in the customary way, using a timer and RC network. And, it provides the necessary wakeup pulse so that the DVR will record for the preset time each time motion is detected. Additionally, it was required that if motion is detected during the recording process, the recording time would "reset" and began the time interval again for each subsequent output pulse from the motion detector.

Theory of Operation

U1 in the drawing sets the recording time via R2 and C1. Several minutes of recording time can be realized with selecting R2 from a few Megohms to a few tens of Megohms (Note: the value given on the schematic is for simulation only). Timer U2 has two purposes: It triggers U1 after about 100mS from the time the pulse is detected, via Q1, and also generates a short "wake up" pulse via C4, which is a pulse of about 50mS. Diodes D1, D2 and output transistor Q3 combine the wakeup pulse and the long timing pulse for the DVR. The connection to the DVR is taken at the collector of Q3. If any pulse is detected from the IR unit, U1 discharges C1 thru Q1, and thus the DVR time is started again from zero. The wake up pulse isn't forwarded to the DVR in that case.

Simulation and Testing: Design and simulation was done by me. Circuit construction and testing was done by the client. The circuit works as described.


DVR IR Motion Detector.gif

Parts List

Dual 555 timer 1ea
47U electrolytic capacitor 4 ea
1-10Meg ohm resistor (for R2) 1 ea *Value for R2 on schematic is for simulation only.
10K 1/8 watt resistors 2 ea
1K 1/8 watt resistor 1ea
1.5k 1/8 watt resistor 1ea
3.3k 1/8 watt resistor 1ea
1n4148 diode or equiv 2ea
2n3904 NPN transistor 2ea
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