non-mechanical SPDT Switch? - Page 1
Forum section: electronics / beginners

author: Falcon69
author_rank: Super Contributor
timestamp: February 15, 2014, 03:22:06 am
content: I am looking for something that is not mechanical. I don't have the room for a mechanical switch. I can't use a relay, as a relay is just two slow.Must be small and SMDI have 3 wires1. +5 volts (won't see anymore then maybe 100mA)2. Ground3. 3rd wire is either connected to ground or positive, based on which side of the switch I want to throw.I want the switch to throw one way, supplying +5 volts to my primary load and cutting off power to the secondary load, when I take the 3rd wire and connect it to wire 2.I want the switch to throw the other way, supplying +5 volts to my secondary load and cutting off power to the primary load, when the 3rd wire is connected to wire 1.Is there anything simple out there that will work?Thanks in advance.Jason

author: Falcon69
author_rank: Super Contributor
timestamp: February 15, 2014, 03:31:14 am
content: Hmmwould this work?http://www.ti.com/lit/ds/symlink/ts5a3159.pdfThe wire going to IN would be +5 volts or ground, depending on which I connect tooV+ and COM would be connected to +5 voltsthat should allow the switch to take the +5 volts connected to the COM between the NC and NO, depending on what IN was connected to.Is this correct thinking?

author: sleemanj
author_rank: Super Contributor
timestamp: February 15, 2014, 07:05:01 am
content: Just a half H bridge.Two mosfets stacked, one P ch, the other N ch. Output at the center. Switch P on and N off = 5v output, switch P off and N on = gnd at output, switch both off = output high z, switch both on = dead fetsBetter to use a half H driver (or use half of an H driver) which protects against the shoot through and includes the mosfets and gate drive internally.

author: Falcon69
author_rank: Super Contributor
timestamp: February 15, 2014, 03:52:27 pm
content: i need 1 input that switches two inputs. Your idea suggests two inputs with one output.I have one wire that i can use as the switch. When it is connected to ground, it will switch the loads, supply power to load 1, when the wires is connected to +5 volts, it will supply the power to Load 2.Basically, I just need somthing that acts like a SPDT switch, but instead of it being mechanical, it will switch based on what I connect wire to, whether ground, or power.

author: Marco
author_rank: Super Contributor
timestamp: February 15, 2014, 04:41:33 pm
content: So is 200 mA max current with 1 Ohm resistance sufficient for your needs?

author: Falcon69
author_rank: Super Contributor
timestamp: February 15, 2014, 04:45:24 pm
content: i will be using 1 op amp on each circuit, 2 transistors, 2 led's, and a hall sensor. there will be less than 100mA total.

author: Marco
author_rank: Super Contributor
timestamp: February 15, 2014, 06:11:30 pm
content: If the voltage drop (100 mV for 1 Ohm/100mA obviously) is not a problem then yeah, a SPDT (analog) multiplexer would work for you ... there's a huge number of them BTW (just search for SPDT on for instance mouser or digikey and then select the multiplexer category and select for parts with supply of 5 Volt or more).

author: Falcon69
author_rank: Super Contributor
timestamp: February 15, 2014, 06:18:14 pm
content: This is what I am trying to doWill this work?This is a switch that will activate when a magnet gets close to the hall sensor. But, I want to be able to choose whether it is a normally open, or a normally closed switch, depending on the application I'll use it for.I am very limited on board size, and that is why I cannot place an actual mechanical switch.

author: IvoS
author_rank: Frequent Contributor
timestamp: February 15, 2014, 08:14:16 pm
content: I think when you switch the LED on and off with the transistor shorting it out then you have about 18.5mA constant current flowing through.I am designing something similar but with linear SS39 hall sensor using LM358 as a comparator and triggering LM555 and then it goes further to logic circuits.

author: Falcon69
author_rank: Super Contributor
timestamp: February 15, 2014, 08:19:21 pm
content: would a diode between the 560 ohm resistor and the 2N2222 resistor stop that? I think I forgot to put that in place.Is that what you are talking about when you are saying the transistor is shorted out?

author: IvoS
author_rank: Frequent Contributor
timestamp: February 15, 2014, 08:30:42 pm
content: It's the first transistor that shorts the LED. Transistor is not shorted out, the LED is. This will work but I think it's impractical, you don't have to constantly draw 18.5mA when the LED is OFF.

author: Falcon69
author_rank: Super Contributor
timestamp: February 15, 2014, 08:40:20 pm
content: I want the green LED to switch off when the hall sensor is triggered, thus the Red LED goes on. Red LED is off when switch is NOT triggered, and Green LED is on.But, I just realized that this circuit isn;t going to work. The Optoccouplers are too large, they won't fit on the pcb.

author: sleemanj
author_rank: Super Contributor
timestamp: February 15, 2014, 11:08:31 pm
content: Quote from: Falcon69 on February 15, 2014, 03:52:27 pmTwo p channel fets, and a transistor inverter to drive one of the gates.Excuse the crudity of the attachment.http://youtu.be/m_JihtALr4EOf course, use N channel if you want to switch low side instead, same gate configuration otherwise.

author: Falcon69
author_rank: Super Contributor
timestamp: February 16, 2014, 01:34:46 am
content: thank you sleemanj, I understandBut, I need this whole circuit to simulate a simple normally closed, or normally open mechanical switch, as the optocouplers would. But, simply, i just need them to switch from one or the other, based on what i have the wire tied to, either ground or power.Which program were you using for that video?

author: sleemanj
author_rank: Super Contributor
timestamp: February 16, 2014, 03:39:30 am
content: Quote from: Falcon69 on February 16, 2014, 01:34:46 amThe simulation is in"CircuitWizard", the recording was withgtk-recordMyDesktop

author: sleemanj
author_rank: Super Contributor
timestamp: February 16, 2014, 03:46:38 am
content: Quote from: Falcon69 on February 16, 2014, 01:34:46 amSo, one fet for the "load" and a way to select if the gate is driven directly (normally closed), or inverted (normally open) from your input signal?

author: Falcon69
author_rank: Super Contributor
timestamp: February 16, 2014, 03:48:54 am
content: yesand thanks, I'll go play with circuit wizard