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Help! I need a timing circuit ASAP!

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A friend of mine has a bear denning on her property, and just noticed that he is getting ready to leave the cave, and will probably do so in just a couple of days. To get a video of it leaving the cave, she bought a video camera (Sony DCR-SR45) and asked me to help her set it up to automatically record the bear whenever it comes out. Because there is no time to order anything online, I picked up the only model of motion detector available in town (Optex RX-40PI), and have been frantically trying to find a timing circuit to use with it and the camera. I can get some 555 chips and (some) discrete components, but probably only very common values. As a motion detector sends out a signal as long as it sees motion, I need the circuit to trip a solenoid (to start the camera recording by pressing the 'record' button) as soon as the MD triggers it, then go into a timing loop, resetting every time the MD sends a signal, until a set period (e.g. 3 minutes) has gone by, when it triggers the solenoid again to stop recording and deactivates until the next signal from the MD.

While I can tell a transistor from a capacitor, and can put together a circuit from a schematic & pinouts, designing such a circuit is beyond my skills.

Here are some further notes that I will add to as I go. I know enough to know that I might not be providing you with enough information, and would be happy to supply whatever you need.
  1. Project will be powered via 'Cat' batteries. (Two in parallel, so that one can be removed for recharging without disturbing the setup. Will that be all right?)
  2. Camera will be powered by charger plugged into inverter.
  3. Camera will be in a security camera enclosure.
  4. Camera can't be physically altered.
  5. Camera has a sliding switch that changes to night vision mode, which would be handy in case the bear shows up at dawn/dusk. I have some wide-angle, bright IR LEDs that can be slaved to add more light to the area.
  6. Solenoid will probably be 12v from an auto wrecker if I can't find anything lighter. (Suggested alternates?)
  7. Any time definite values aren't required, ranges (e.g. 100-470Ω) would be preferred, as local sources are slim.

If any of you could post a schematic to such a circuit, I (and she) would be extremely grateful.
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