## simple long range transmitter circuit suggestion

 simple long range transmitter circuit suggestion From: - 2003-06-30 07:45:13 ```Hi, I was looking at the circuit http://www.lirc.org/images/better_transmitter.gif I thought it seemed like a waste of current with that first NPN transistor -> 100 ohm resistor -> two forward diodes -> ground, driving the BD237. Would it be possible to use just a single MOSFET instead? Like a 2N7000? - Id (drain current) = 300mA - Idm (peak drain current) = 1.3A - Vgs(th) (gate-source threshold) = 2V (http://my.semiconductors.com/acrobat/datasheets/2N7000-03.pdf) Which seems to suit the IR transmitting LED I want to use. - If (forward current) = 50mA max - Ip (peak forward current) = 1.2A - Vf (forward voltage) = 1.2V @ 20mA http://www1.jaycar.com.au/productView.asp?ID=ZD1945&CATID=&keywords=infrared&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=xxxxxxxxxx&Keyword2=xxxxxxxxxx&pageNumber=&priceMin=&priceMax=&SUBCATID= I studied over the datasheet, and brushed up on MOSFETS (http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/mosfets.shtm http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/mosfet.html) Now, can someone please tell me if my following calculations are right? Using this circuit. +12V o----------+ | _|_ \ / \ (IR LED) --- \ | | | D Signal G |-+ (DTR) o--------| 2N7000 (MOSFET) |-+ | S | | | GND o----------+ And given the non-saturated region of the MOSFET is when Vds < Vgs - Vt Here Vds = 12V (of power supply) - Vf (of LED) = 12 - 1.2 = 10.8V and Vgs = 12V (when the serial port puts out 12V as a high) ie. 10.8 > 12 - 2 => The MOSFET will be saturated. Since the duty cycle is 25 us (for the 40 kHz carrier frequency used) Looking at Fig. 3 from http://my.semiconductors.com/acrobat/datasheets/2N7000-03.pdf I read off the graph that Id will be 1A. (Which is just below the peak forward current of 1.2A allowable for the LED - hopefully this will give a good long range.) And when Vgs is below Vt (2V) the MOSFET will be off. (Ie. when the serial line goes to its low of -12V) Also, will I need any resistors and/or capacitors in there to handle the dynamic charactersitcs? Chris. http://mobile.yahoo.com.au - Yahoo! Mobile - Check & compose your email via SMS on your Telstra or Vodafone mobile. ```

 simple long range transmitter circuit suggestion From: - 2003-06-30 07:45:13 ```Hi, I was looking at the circuit http://www.lirc.org/images/better_transmitter.gif I thought it seemed like a waste of current with that first NPN transistor -> 100 ohm resistor -> two forward diodes -> ground, driving the BD237. Would it be possible to use just a single MOSFET instead? Like a 2N7000? - Id (drain current) = 300mA - Idm (peak drain current) = 1.3A - Vgs(th) (gate-source threshold) = 2V (http://my.semiconductors.com/acrobat/datasheets/2N7000-03.pdf) Which seems to suit the IR transmitting LED I want to use. - If (forward current) = 50mA max - Ip (peak forward current) = 1.2A - Vf (forward voltage) = 1.2V @ 20mA http://www1.jaycar.com.au/productView.asp?ID=ZD1945&CATID=&keywords=infrared&SPECIAL=&form=KEYWORD&ProdCodeOnly=&Keyword1=xxxxxxxxxx&Keyword2=xxxxxxxxxx&pageNumber=&priceMin=&priceMax=&SUBCATID= I studied over the datasheet, and brushed up on MOSFETS (http://www.powerdesigners.com/InfoWeb/design_center/articles/MOSFETs/mosfets.shtm http://www-g.eng.cam.ac.uk/mmg/teaching/linearcircuits/mosfet.html) Now, can someone please tell me if my following calculations are right? Using this circuit. +12V o----------+ | _|_ \ / \ (IR LED) --- \ | | | D Signal G |-+ (DTR) o--------| 2N7000 (MOSFET) |-+ | S | | | GND o----------+ And given the non-saturated region of the MOSFET is when Vds < Vgs - Vt Here Vds = 12V (of power supply) - Vf (of LED) = 12 - 1.2 = 10.8V and Vgs = 12V (when the serial port puts out 12V as a high) ie. 10.8 > 12 - 2 => The MOSFET will be saturated. Since the duty cycle is 25 us (for the 40 kHz carrier frequency used) Looking at Fig. 3 from http://my.semiconductors.com/acrobat/datasheets/2N7000-03.pdf I read off the graph that Id will be 1A. (Which is just below the peak forward current of 1.2A allowable for the LED - hopefully this will give a good long range.) And when Vgs is below Vt (2V) the MOSFET will be off. (Ie. when the serial line goes to its low of -12V) Also, will I need any resistors and/or capacitors in there to handle the dynamic charactersitcs? Chris. http://mobile.yahoo.com.au - Yahoo! Mobile - Check & compose your email via SMS on your Telstra or Vodafone mobile. ```
 Re: simple long range transmitter circuit suggestion From: Blars Blarson - 2003-07-01 03:20:02 ```In article <20030630074504.42032.qmail@...> ottrey@... writes: >I was looking at the circuit >http://www.lirc.org/images/better_transmitter.gif You might also be interested in http://www.blars.org/blarslirc/ for an alternate circuit. The diodes and second transistor make a constant-current sources, so the fact the voltage in the capacitor droops doesn't matter. Your circuit would have very inconsistant light pulses if line-powered, and if you are using extenal power why are you worrying about a few miliwatts on a low duty cycle? -- Blars Blarson blarson@... http://www.blars.org/blars.html "Text is a way we cheat time." -- Patrick Nielsen Hayden ```

No, thanks