4.5v latching relay one button driver - Page 1
Forum section: electronics / beginners

author: autumn23
author_rank: Newbie
timestamp: August 13, 2013, 07:43:31 pm
content: Hi all, i have a project that i need some help on. I need to be able to run 4 panasonic 4.5v one coil latching relays. The kicker is i need to turn each on and off with one button i.e. channel one has its own relay and momentary switch, channel two would have its own relay and momentary switch, and so on. I've looked into using an H bridge but i want to try to keep the parts down, is there a better way of controlling the relay. Thnks here is the datasheet for the relays [http://pewa.panasonic.com/assets/pcsd/catalog/tx-s-catalog.pdf/url]

author: Zero999
author_rank: Super Contributor
timestamp: August 13, 2013, 08:23:53 pm
content: What sort of switches are you using?Ideally you need SPDT so you can make an H-bridge from the switches.If you only have SPST contacts, see the application note and replace the MOSFETS inside the IC with the switches.http://www.maximintegrated.com/app-notes/index.mvp/id/3288

author: autumn23
author_rank: Newbie
timestamp: August 13, 2013, 09:16:26 pm
content: I planned on using n/o momentary foot switch. I also would like to have one button do both functions, set and reset of the relay. I saw a video on YouTube that used mosfets and npns to latch but it switches between v+ and no voltage I need to switch between v+ and v-

author: Psi
author_rank: Super Contributor
timestamp: August 13, 2013, 10:26:04 pm
content: Probably not what you want to hear but this is the sort of situation where i throw a ~$0.50 microcontroller at the problem.4.5V @ 11mA you can drive from the i/o port directly.- Cheap micro using- 1 input pin for your n/o foot pedal switch (or 4 inputs if you want separate control)- 8 outputs (2 for each relay so you can flip polarity).- 2min of coding.- 4 backemf diodes.Done.

author: autumn23
author_rank: Newbie
timestamp: August 13, 2013, 11:10:13 pm
content: Awesome! Quick, cheap, easy, I love it. Can you recommend an ic

author: Simon
author_rank: Unknown
timestamp: August 14, 2013, 05:37:06 am
content: any MCU with 12+ digital I/O's will do the job

author: Psi
author_rank: Super Contributor
timestamp: August 14, 2013, 07:24:07 am
content: Actually, due to the relays needing both polaritys, the protection diodes will have to be zeners (2 per relay) or better yet, one bidirectional TVS diode per relay.Hm.. this is an interesting issue. I've never had to use single coil latching relays before.Do you even need diodes when driving the coil from two push/pull outputs?

author: fcb
author_rank: Super Contributor
timestamp: August 14, 2013, 10:17:16 am
content: Quote from: Psi on August 14, 2013, 07:24:07 amYou can slug a relay-coil with a capacitor to lower the size of the back EMF pulse, you might end up with an interestingly tuned circuit though.

author: Zero999
author_rank: Super Contributor
timestamp: August 14, 2013, 01:11:09 pm
content: Quote from: autumn23 on August 13, 2013, 11:10:13 pmWhat about the 74HC74?You should be able to control two relays per IC.It has the advantage of not requiring any programming but the disadvantage of drawing more power because the coil would be powered continuously or you could try putting a capacitor in series with the relay coil so it's only powered briefly on transitions.http://www.electronics-tutorials.ws/counter/count_1.htmlThe input frequency comes from the switch - use a 1M pull-down and put a 100nF capcitor across the switch for debouncing.Connect the relay coil between Q and Q', with some diodes for back-EMF protection.

author: autumn23
author_rank: Newbie
timestamp: August 14, 2013, 02:33:10 pm
content: I'm looking into the atmega48 chip. I need to do some pwm as well as control the relays. The atmega48 should handle both