EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: GeoMetry on June 20, 2024, 05:39:49 pm
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I've been working on this for a few days. I started with a "complex" requirement that I tried to break down into smaller pieces. I am looking for an elegant solution but at this point anything that works is going to make me happy. The basic requirement is for a circuit the flashes lights and beeps a buzzer for five seconds then just illuminates the light bulb after that. I have a circuit using a 555 timer astable to do the beeping and flashing, the other circuit that just illuminates a bulb is obviously simple. What I am having trouble with is switching from one circuit to the other after 5 seconds. I think I would use another 555 timer and some sort of latching but I can't seem to come up with anything that works. I'm pretty sure there is a more elegant solution out there but I needed to break it down this way in order to try to find a solution with my limited knowledge.
Complete details: This is part of a seat belt warning system in an automobile so it is 12VDC negative ground. This circuit has three 12VDC inputs and a switched ground. The first input is from the battery through a 10 amp fuse. The second input is through the 1.5 watt warning light bulb. The third input is through the buzzer. The connection to ground is completed when the seatbelt is unbuckled. When the seatbelt is buckled the connection to ground is broken. Once the seatbelt is buckled the circuit is broken and the light and buzzer both stop. If the seatbelt is subsequently unbuckled the process starts over.
Thank you
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Are you modifying an existing seatbelt warning signal? If yes, proceed with extreme caution: the seatbelt sensor controls airbags in many modern cars. More precisely: the airbag is meant to fire only if the seatbelt is buckled. If it’s not, but the car thinks it is, during a collision a person will have their head or hands obliterated by airbag explosion.
I don’t know, how your circuit looks now. But the simplest timing circuit would be just a capacitor charged through a sufficiently big resistor. The output controls a transistor, that cuts off power to the buzzer circuit. Use a Schmitt trigger or another transistor (common-emitter) to ensure quick transition. Remember about pull-up or pull-down resistors (depending on circuit orientation) to ensure all elements are initially in the right state and the capacitor can easily discharge.
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No airbags, 35 year old car and I think the capacitors have destroyed this PCB. After getting 5 others from junkyards I decided to go this way. I'm trying to replace the PCB and recreate everything it does. It only has 5 simple functions. This is my last hurdle. I have breadboarded the other functions but I can't even come up with a design for this.
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Are you modifying an existing seatbelt warning signal? If yes, proceed with extreme caution: the seatbelt sensor controls airbags in many modern cars. More precisely: the airbag is meant to fire only if the seatbelt is buckled. If it’s not, but the car thinks it is, during a collision a person will have their head or hands obliterated by airbag explosion.
That's 100% not true. Airbags are absolutely intended to fire when the seatbelt is not buckled, and crash testing is required in both configurations. Advanced airbags are controlled by weight sensors in the seat. No weight or very light weight == no deployment, due to an empty seat or a small child that would be more likely injured by the airbag than helped. Medium-light weight has reduced deployment speed for larger children or smaller adiults, and larger adults get the full deployment force. They may also modify the deployment rate based on the seatbelt, so defeating the seat belt sensor in a car with airbags isn't a good idea, but the idea that airbags aren't meant to work when people don't use their seatbelts is completely wrong.
Also, while people can be injured by airbag deployment, they are only "obliterated by airbag explosion" if they have an unfixed Takata airbag that has become sensitized by moisture. If you still have one of these, especially the "alpha" airbags (mostly older Honda airbags) get it replaced immediately. If your vehicle has the alpha airbag, literally do not drive this vehicle except to the dealer to have it replaced or disabled.
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here is the circuit I designed. But I'm a novice and I assume I have screwed this up and would appreciate being corrected. If this is just a silly way to solve this problem I would be very happy to learn about a more sensible solution. The red box in this schematic corresponds to the red box in the picture in my first post.
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