Active low transmitter question

[peterdeco1]

I agree Richard. It's like putting a Ferrari engine on a lawnmower. At work we have a small PCB with an 8-pin PIC and 4 of these mosfets. It replaces about a dozen different subassemblies in products all governed by the PIC program. These transistors sometimes simply switch a string of LED's on and off, a 2A DC motor or a combination of the two. That's why I am "programmed" to using them.

[LEDave]

"what relay ? specs ?"

Hi Richard - I haven't got around to choosing one as yet.

I need a relay activated by the pic (3.3v) via a transistor/mosfet to switch an alarm sounder on (fairly loud sounder) probably 12v (I haven't chosen the sounder yet either).

I think I need to try and do a better schematic to show what I'm trying to achieve - I'm a carer, really busy right now but will try and do a more appropriate diagram over the weekend and my thinking on the components.

Thanks for your help as ever.

Dave

[peterdeco1]

I'm sure by "spec's" he means how much current does the relay draw.

[LEDave]

Ok - Instead of a schematic (can't find all components in menu I need to make drawing), have decided to talk through each section of the project as there isn't really that much to it if that's ok?, so:

Section one / Transmitter Side (Battery Operated): Through beam sensor (which I have bought along with the transmitters) http://www.alibaba.com/product-detai...2abf516dBZICkn Half way down page under 'Retro Reflective Type'

The sensor is 12v and operates over a distance of 5m in daylight, beam gets broken and outputs 12v (obviously too high for a input into the pic) so to drop the voltage down to 3.3v I'm thinking of a Voltage divider Vout = Vs x R2 / (R1+R2). Vs = 12v R1=1k R2=390 Ohms giving Vout of 3.367v or a 12v to 3.3 Zener cct or using a http://uk.rs-online.com/web/p/voltag...4923/?sra=pstk (not sure on which way to go).

So the beam gets broken - 12v from sensor down to 3.3v to the pic, pic reads the input pin as high and makes an output pin change state and causes the transmitter to Tx.

Section two / Receiver side (mains operated): The receiver receives a Tx input, pin on pic reads the Tx input and sets an output pin high onto the gate of a 2N7000 http://uk.rs-online.com/web/p/mosfets/6714733 which then drives either a http://uk.farnell.com/axicom-te-conn...55?st=3v relay or a http://uk.rs-online.com/web/p/solid-...relays/8887619 which in turn drives a loud sounder http://uk.rs-online.com/web/p/piezo-...E&gclsrc=aw.ds

I have been drawing a pcb layout for these cct's using Diptrace as I'm going to make my own / first pcb's too.

Any thoughts on the above as ever very much appreciated as ever.

[richard]

which in turn drives a loud sounder

why a relay at all ? the 2n7000 could drive that alarm directly with ease

[LEDave]

why a relay at all ? the 2n7000 could drive that alarm directly with ease

I never thought of that, great idea, cheers Richard.

What do you think the best way to drop the 12v from the sensor output to 3.3v into the pic, would the voltage divider be ok/work?

[richard]

works for me. you could protect against divider failure with a 3 to 5v zenner if you are paranoid
the first pic may be unnecessary too if the tx works like i think it does