Receiver with TTL outputs, how to drive a piezo sounder and an LED?

Thread Starter

Chris Wilson

Joined Dec 2, 2011
17
Beginner question, please bear with me!

I have a miniature receiver that has 4 TTL outputs, each independently triggered by one of 4 channels from a paired transmitter. Each output is TTL and rests low, and is driven high.

I want each TTL output, when set high by the corresponding channel on the TX being heard, to both light an LED and sound a 3V 40mA piezo sounder. One sounder and one LED is dedicated to each TTL RX output. The receiver and transmitter both run on 2 to 3.6V at minimal current. I have a mains powered 3.3V step down supply at 5W for the TX and another for the RX. It would be ideal to use the same supply to drive the piezo sounder and an LED per channel for the receiver.

I am thinking MOSFET to do this but would need a basic schematic with values and a comfortably excessively rated MOSFET part number.

Would anyone be kind enough to help me here please? A concern is any implications in using the same supply for the RX itself and what its TTL outputs drive.

The RX datasheet should be linked here: https://www.rfsolutions.co.uk/downloads/62822381d8de6982DS-ZPT-3.pdf

Thanks.
 

MrChips

Joined Oct 2, 2009
27,679
Here is a generic circuit schematic on how to configure a BJT or MOSFET driver.

1669068139084.png


The TTL logic represents your receiver module. Ignore the +5v supply. Your supply voltage and +VDD will be +3.6V.
The DC Motor M is your buzzer.
RG = 1kΩ
R2 = 10kΩ

Instead of the snubber circuit shown, turn the diode around and make this your LED. Select the series resistor (about 100-470Ω) for desired brightness.

For MOSFET, I'm thinking 2N7000 should do. Ignore D1.
 

Irving

Joined Jan 30, 2016
3,190
I'd use a simple quad driver such as the MIC4467. Though slightly more expensive than individual MOSFETs it has the advantage that it is fully characterised for a TTL input and therefore will give guaranteed results and will sink upto 1.2A per output (total package limited) Also the DIP/SOIC packaging and reduced component count make it simpler to use than discrete parts.

While @MrChips generic solution is a valid approach, the 2N7000 MOSFET may not be the best choice. At a TTL high input of 2v its turn-on is marginal, given the buzzer needs about 20mA and the LED 10mA (it really needs 3.5 - 4v). You'll have to try a few, possibly from different manufacturers, to find some that work OK at 2v.
 

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