0
\$\begingroup\$

I got a new HTC one m9 and it has an FM reciever in it, it picks up all the radio channels in my area really well. I am trying to make a very simple radio transmitter for the fun of it. I dot want something complicated that transmits voice, I want to transmit a simple beep, pause, beep, pause etc. I dont have a lot of capacitors so I dont want to make anything complicated. I found this schematic online and built it:The schematic

Instead of 1.5 volts i have close to 7 volts though. The site just said it was an RF transmitter(radio frequency) Should my Htc FM reciever be able to pick up anything from this, if so how can I make a noise of anysort that i can recognise it? Thanks.

\$\endgroup\$
  • \$\begingroup\$ jbgizmo.com/page21.html often NPO caps (pF) are needed with tiny precision tuner variable cap from old portable radio or sites that sell such parts \$\endgroup\$ – Sunnyskyguy EE75 Jan 7 '17 at 16:23
  • 1
    \$\begingroup\$ Hmmpf. Nothing useful about how it works nor at which frequency it transmits on the original site. You would probably do better to find a circuit that has a description that at least says at which frequency the transmitter operates. It would also be nice to know what it transmits. \$\endgroup\$ – JRE Jan 7 '17 at 16:27
  • \$\begingroup\$ There are better ways but requires understanding of chips using Xtal and harmonic oscillator then using a diode to vary frequency with a simple tone gen. with transistor or CMOS HC4060 clock and divider to get on/off cycle and tone in one chip to drive varicap to shift FM osc. such as 35MHz PLL using VCXO shifted by 25kHz then multiplied x3 (harmonic) to get 85~90 or 104~106MHz band or away from broadcasters digikey.com/product-detail/en/texas-instruments/CD74HC4046AM96/… Keep in mind there are 4 variables . FM centre, deviation, tone f and On Off f \$\endgroup\$ – Sunnyskyguy EE75 Jan 7 '17 at 16:31
  • 2
    \$\begingroup\$ that's a mighty strange battery symbol: anyone care to guess what polarity it should have? Hint: the "+" and "-" symbols make sense. \$\endgroup\$ – glen_geek Jan 7 '17 at 16:37
  • \$\begingroup\$ More details about circuit construction/operation from here talkingelectronics.com/projects/Spy%20Circuits/… \$\endgroup\$ – JIm Dearden Jan 7 '17 at 18:48
2
\$\begingroup\$

Your circuit can actually oscillate, and might even oscillate at a frequency within the FM band. It is a minimum-parts-count circuit that can fail to oscillate from too many causes. For example, the battery is in part of the circuit that is oscillating, and its stray coupling to adjacent parts will greatly affect frequency. It is not a circuit that will yield repeatable results.
Even if oscillating, and oscillating at the correct frequency, it will not produce your desired tone in an FM receiver. It will merely produce silence. You need an oscillator with provision for frequency or phase modulation, and an audio oscillator to provide that modulation. an example tone transmitter

A transmission-line oscillator is shown below that uses no coils or small-value capacitors. Frequency is set by the length of twisted-pair transmission line: a one-quarter wavelength is desired ideally, but a shorter length yields inductance that combines with gate capacitance for resonance. You might start with a length slightly longer, so that you can trim the length (and re-solder the ends) to raise frequency. For FM broadcast band, a length of about 15 inches might be a good starting point (its been awhile, my memory of lengths & frequencies is uncertain). DC supply voltage to the chip is not shown. Use no more than five volts, and include a 0.01uF capacitor with very short leads from supply pin to ground pin.
This is a single-frequency oscillator - I have not tried to frequency-modulate it, but superimposing an audio-frequency signal on the Vdd supply voltage should result in FM.transmission-line oscillator

\$\endgroup\$
  • \$\begingroup\$ Do you know of any schematics that use few capacitors, or capacitors that are not picofarad or lower? \$\endgroup\$ – Jelly Jan 7 '17 at 16:39
  • \$\begingroup\$ What would happen if I changed the picofarad capacitors with say, a nano or microfarad capacitor or some value? \$\endgroup\$ – Jelly Jan 7 '17 at 16:45
  • \$\begingroup\$ @Jelly Any LC oscillator does require small picofarad capacitors, and in combination with stray capacitance is most often a cause of failure to oscillate at the correct frequency. A delay-line or transmission-line oscillator can be built using no picofarad capacitors. A quick web search came up empty. \$\endgroup\$ – glen_geek Jan 7 '17 at 16:51

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.