Help with Combining NE555 Light Sensor and Monostable Circuit - Page 1
Forum section: electronics / beginners

author: OxideXen
author_rank: Newbie
timestamp: November 26, 2024, 07:38:16 am
content: Hi everyone,I'm working on an assignment to make an automatic curtain system. I'm using an NE555 circuit to sense light or darkness and a monostable multivibrator to control a motor, so it runs for a specific duration.My question is:How can I combine these circuits effectively to drive the motor? Should I directly connect the output of the light sensor circuit to the trigger of the monostable circuit, or is additional circuitry required?

author: Terry Bites
author_rank: Super Contributor
timestamp: November 26, 2024, 09:47:40 am
content: I'm assuming that is a real comparator not an opamp.Some opamps might work but its best to use something like an LM393.With an opamp you dont need a "pull up" on the outputsYou need alow going edgeto trigger the 555.Your circuit as it stands provides a high going edge.No worries, just rearrange the comparator polarity as shown.Add a little "hysteresis", look that up. It stops the circuit flapping about when the LDR is on the cusp of detection.You might want to experiment with the positive feedback resistor value.I reccomend driving the LED using an extenal transistor whe using an "open collector" comparator.If you dont, it wil mess with the open collecor voltage. A pnp 2N3906 is very cheap. As shown in my schematic the LED will light up when the comparator output is low.

author: PGPG
author_rank: Frequent Contributor
timestamp: November 26, 2024, 12:05:03 pm
content: Quote from: OxideXen on November 26, 2024, 07:38:16 amI have never heard of using NE555 timer to sense light.Quote from: OxideXen on November 26, 2024, 07:38:16 amMore typical is to have limit switches at the end of movement. So from sensor you have permanent signal specifying if it should be open or close and that makes motor running in specified direction until limit switch will stop it.

author: OxideXen
author_rank: Newbie
timestamp: November 26, 2024, 12:49:22 pm
content: Quote from: Terry Bites on November 26, 2024, 09:47:40 amahh yes, sorry i forgot it is an LM393so how do you make it a low going edge to connect it to trigger the 555? i'm still confused on how to do itQuote from: PGPG on November 26, 2024, 12:05:03 pmyeah i also thought that a mechanical option would be an easier route, but is it possible to do it like i originally intended?

author: janoc
author_rank: Super Contributor
timestamp: November 26, 2024, 02:03:10 pm
content: Quote from: OxideXen on November 26, 2024, 12:49:22 pmPossible yes, good idea - no. Consider what happens when the curtain is somewhere in the middle instead of fully open or fully closed, for example because it has been moved manually or because it didn't move completely to the end last time (555 timers are not exactly accurate). Your circuit has no idea about that and will attempt to drive the motor for a fixed time regardless, possibly damaging the mechanism or burning the motor out. Or both.

author: OxideXen
author_rank: Newbie
timestamp: November 26, 2024, 02:30:56 pm
content: Quote from: janoc on November 26, 2024, 02:03:10 pmoh yeahh, you're right. so i do need to use a limit switch. thanks

author: Ian.M
author_rank: Super Contributor
timestamp: November 26, 2024, 02:45:04 pm
content: ... or a slipping clutch or other friction drive.However its *much* easier to simply use limit switches (typ. heavy duty microswitches) bypassed by diodes (so the motor can reverse away from the limit) in series with the reversible DC motor, driven by a H-bridge or other polarity reversing drive circuit, which could be as simple as a DPDT switch or relay. It simply runs till the motor reaches one of the limits, where the limit switch opens the circuit. The same effect can be achieved with low current limit switches or sensors that inhibit the drive circuit when activated.However to avoid motor burnout if something obstructs the mechanism mid travel, its still worth having either a timer circuit to cut power after typically double the time to open/close or load current sensing to detect jams and trip till its power-cycled.

author: RJSV
author_rank: Super Contributor
timestamp: November 26, 2024, 10:15:27 pm
content: I believe your question on how to get reversed output, would be solved by simply reversing the 2 inputs. The hysterisis signal still connects to the + input.Looks like you are learning at least 3 things all at once. Should be able to split things up, and just learn 1 at a time. But there is maybe a teacher and curriculum in this, so might have live with doing all at once.

author: Zero999
author_rank: Super Contributor
timestamp: November 26, 2024, 11:23:27 pm
content: I would use limit switches as mentioned above. A PTC resistor (polyfuse) can be used for over current protection.For educational purposes only, a comparator can be used to make a monostable.