Digital Switch - Page 1
Forum section: electronics / beginners

author: Jasonbit
author_rank: Regular Contributor
timestamp: September 01, 2018, 12:10:40 am
content: Hello folks,I need a component to serve as digital switch (I don't know if this is the correct name). I need something which I activate through signal MCU and this component establish the connection on a given wire.I need some with a low resistance through contact and a higher impedance for when I put a high VCC level in contact it offers a higher level of resistance even though it is open. Also I need a component with large contact, because I will need command about 40 wires.I searched through various articles and I found this component "HCF4066B" (http://pdf.datasheetcatalog.com/datasheet/SGSThomsonMicroelectronics/mXwvuw.pdf).What do you think about this component? Which type component should I search? I am looking for "digital switch component".Thanks

author: Cerebus
author_rank: Super Contributor
timestamp: September 01, 2018, 12:41:53 am
content: As always - masses of information missing from the question. Sorry, but I get a bit tired of this and wish people would just take note of how other questions have come off the rails by omitting basic relevant information before plunging in and asking their own. Anyway, grumble over...What voltage ranges are you talking about (0-5V, 0-20kV,...) , what current ranges are you taking about ( 0-10 uA, 0-100 A,...) , DC or AC, if AC what bandwidth?A 4066 will be fine for medium speed digital signals or lowish level analogue signals (0-5V, possibly higher depending on supply voltages, a few mA) but bugger all use if you want to drive a one horsepower AC mains motor from it.

author: senso
author_rank: Frequent Contributor
timestamp: September 01, 2018, 12:49:31 am
content: Relay, TRIAC, opto-isolator, transistor, mosfet, SSR, and some/lots of multiplexers, CD4066, adg333, max333, adg201/202, ULN2003/2803, TPIC6B595(and the A and C variants), and many more things might be able to achieve what you want.But, voltage, current, isolated or not, needs to be known..

author: Jasonbit
author_rank: Regular Contributor
timestamp: September 01, 2018, 01:29:39 am
content: Quote from: Cerebus on September 01, 2018, 12:41:53 amQuote from: senso on September 01, 2018, 12:49:31 amhoo sorry but I forgot describe the main functionality to this circuit. As I said I intend through MCU switch on or off a contact where this contact actuate a various loads, like leds, LCD 16*2 (signal and power), keyboard matrix.Therefore the values of loads will prowl 0-5VDC and maybe 200mA maximum.I am reading the datasheet and I highlight these points:1-SWITCH ON RESISTANCE MATCHED TO WITHIN 5Ω TYPE. OVER 15V SIGNAL INPUT RANGE - (these means which the resistance represent ever 0.33% of voltage? E.G. to 5VDC I will 16mΩ?2-EXTREMELY HIGH CONTROL INPUT IMPEDANCE (control circuit isolated from signal circuit 10^12 Ω typ.) - (these points it is good, right? Because this means that the circuit will offer a higher resistance on input pins "IN/OUT", right?3-QUIESCENT CURRENT SPECIF. UP TO 20V - (what means this point?);ThanksFor example, on this article:https://www.insidegadgets.com/2014/06/24/building-a-kvm-part-2-pcb-prototype-and-expanding-past-2-kvm-ports/he used CD4066 to apply as digital switch. But he studied another options but for what I remember he chose the CD4066.

author: Cerebus
author_rank: Super Contributor
timestamp: September 01, 2018, 02:25:36 am
content: Quote from: Jasonbit on September 01, 2018, 01:29:39 amNo, it refers to what it says, thematchingof the on resistance (Ron) of one switch in the IC package to the on resistance of the other switches in the package, how close in value they will be. I have no idea what you think is going on with "0.33% of voltage? E.G. to 5VDC I will 16mΩ" but it suggests to me that you have either some serious misunderstanding of the very basics (i.e. Ohm's law) or great difficulty in expressing yourself (I take that you're not a native English speaker, even though you appear to be resident there).QuoteErm, yes, sort of. It's alluding to the loading effect of the inputs on the input signals (i.e. from the MCU). With CMOS microprocessors the inputcapacitanceis usually the parameter that one would worry about and which has a much greater effect on loading that the input resistance of a CMOS gate which is typically very high. I doubt it is anything you need to worry about too much at the moment, but it wouldn't hurt for you to research the concept of "fan out" for digital circuitry.QuoteJust that, that the quiescent current for the chip isspecifiedfor all supply voltages up to 20V, that is, it is notunspecified at any point. Don't worry about it - presumably some datasheet(s) for this part from some other manufacturer(s) left Iqunspecified under some conditions and the marketing department at SGSThompson decided it was worth pointing out that they had fully characterised the chip. That block of bullet points at the start of almost every datasheet is nearly always just marketing BS and should be ignored in favour of whatever the rest of the datasheet actually says.The Ronof thex4066 type of analog switch is pretty high. You can see from the datasheet that with a 5V supply it has the potential to be as high as 1k ohm. That's fine for a high impedance signal line, but if you're pushing any current worth mentioning it will be a problem (e.g. at 1mA you'll see a whole 1V drop across it). This chip isn't really designed for switching anything more than low level signal lines. If you're looking at 100mA then you're probably in the region where solid state relays are your best bet. In the hinterland between you can probably get what you want with a logic-level MOSFET, but reading between the lines I suspect that you're at the stage where designing with discrete components is going to be problematic for you.If you're looking at switchingbothlow level signalsandsignals with significant current then you're unlikely to find an economic "one size fits all" solution.

author: ocset
author_rank: Super Contributor
timestamp: September 01, 2018, 11:00:47 am
content: i think OP should just try a MOSFET switch.....eg a P mosfet in the line and switch it on with a signal BJT by pulling its gate down.

author: Zero999
author_rank: Super Contributor
timestamp: September 01, 2018, 03:20:36 pm
content: Please post a schematic. As treez suggested, a simple MOSFET switch could do the job, but it depends on whether the load is connected to the 0V, +V or if current needs to flow in both directions.