close a circuit based on a 3.3v dropping low? - Page 1
Forum section: electronics / beginners

author: NCC74656
author_rank: Newbie
timestamp: May 14, 2022, 04:34:53 pm
content: im new to all this so would prefer to do things with hardware if possible. have not learned to code an arduino or such yet.i have a project with a 3.3v reference that pulls down to 0.5v under a condition that i would like to trigger a relay (close a switch) at this time. id like something to sense the voltage and when it drops below 1v to close the circuit, then open again when the voltage goes above the 1v mark (or there abouts).what would i use for this? a PLC? op amps? it seems simple enough but im just not sure where to start

author: tooki
author_rank: Super Contributor
timestamp: May 14, 2022, 04:59:20 pm
content: A comparator is the classic circuit for comparing one voltage to another.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: May 14, 2022, 05:13:32 pm
content: LM393 is an often used comparator and it costs a dime or two (Depending on variant). So that is a place to start.As with all electronics, if it's connected to "external" stuff, connectors or long wires then you may need extra stuff such as filtering and ESD and EMC "stuff" and make it robust against over voltages at it's inputs. But they're cheap enough to buy a handful and start experimenting on your breadboard.

author: David Hess
author_rank: Super Contributor
timestamp: May 15, 2022, 02:34:32 pm
content: 3.3 volts and 0.5 volts are comfortably within the ranges of low and high for TTL signal levels, so a simple logic gate could be used to detect the level.If you use a comparator, then a separate reference level will be required.

author: SmallCog
author_rank: Frequent Contributor
timestamp: May 16, 2022, 02:01:38 am
content: Being the dodgy bloke I am I'd try hooking a suitable solid state relay up to it, if it behaves then happy days!Note that whilst they may switch nicely with a low DC signal not all of them are happy switching DC but are rather intended for switching AC.Choose one that is appropriate to your load, as well as your trigger. Normal relays are simpler in this regard...This is an example of one that will trigger happily from 3VDC but is only suitable for switching AChttps://www.sparkfun.com/products/13015This is one that will switch nicely off of 3V but will be OK with switching a DC circuithttps://au.element14.com/crydom/dra1-cmx60d10/ssr-din-module-60vdc-8a/dp/1784035

author: Terry Bites
author_rank: Super Contributor
timestamp: May 16, 2022, 12:55:20 pm
content: You might use an opamp or a comparator- finding the right opamp pot luck.I have attached three versions of the same circuit- one with a an open drain comparator, one with an opamp and one with a push pull output comparator.The thresholds are 1V and 3V ish. As an exercise, analyse how the the feedback network operates. Assume the comparator/ opamp output swings rail to rail.The driver transistor needs to be able to supply the relay coil current- you may need something other than shown.Put a cap right on the coil's V+. This helps to keep nasty transients off of your V+rail. You can run the relay coil from a higher voltage if you prefer.How much current do you need to switch, AC or DC?3.3V relays have low current contacts. 2A is pretty much the max.An SSR can be driven in the same way from the driver transistor but check the SSR data sheet with regards to control current.SSRs are very expensive compared to an electromechanical relay.

author: antenna
author_rank: Frequent Contributor
timestamp: May 17, 2022, 02:56:31 am
content: Why not find a schmidt trigger with those threshold voltages?? Then you don't have to build a circuit around a comparator and deal with reference voltages etc

author: Terry Bites
author_rank: Super Contributor
timestamp: May 17, 2022, 01:31:34 pm
content: any suggestions?

author: antenna
author_rank: Frequent Contributor
timestamp: May 17, 2022, 04:15:45 pm
content: If all the OP wants is to flip a switch, it really does not matter the voltages and comparing them, and if it is already coming from a logic, then it probably does not need debouncing, so why not a pair of 2N3904 transistors?

author: Terry Bites
author_rank: Super Contributor
timestamp: May 18, 2022, 10:08:03 am
content: If its just switching then something such as 74LV14A or a similar single gate eg 74HC1G14 will be fine.If you have a 7555 knocking about, join thresh to trig and you'll have a Schmitt with thresholds at 2/3 Vdd and 1/3 Vdd.Connect via 10k to the base of a lowly npn- job done. BTW a status LED is always a good idea. LED+Resistor across the SSR inputs