Antenna powered 555 - Page 1
Forum section: electronics / projects

author: hobbyist
author_rank: Newbie
timestamp: February 04, 2017, 05:14:53 am
content: I would like to make a kind of home made RFID device using a 555 timer chip that is powered by an antenna. It would use a full size magnet mount antenna for the 145 MHz amateur band. I would transmit on a handheld radio to power the circuit. I want to use an LMC555 chip, which needs 1.5 volts.I want to make the LMC555 do one of the following, depending on the available power:-- Make a square wave on an oscilloscope.-- Make the needle of an analog multimeter wave back and forth. The meter will be set to the 1 volt range.-- Generate an audio tone that can be heard in sensitive high impedance headphones. (I've got them.)-- Oscillate at 400 KHz into a short wire antenna. A nearby amateur radio set to CW mode will beep whenever the LMC555 is running.-- Make an LED blink.I wired up an LMC555 on a breadboard. It runs fine with a 1.5 volt battery, and draws 650 nanoamps in astable mode. As wired, It makes a 500 Hz tone that is easy to hear in my heaphones.I connected a Schottky diode and a capacitor to the antenna lead. My oscilloscope seems to show that if I use a 47 ohm terminating resistor, I can get 2 volts of DC. Without the terminating resistor, the output looks like a sine wave, not DC. The antenna seems to receive enough power to make the chip run, even though most of the power is wasted in the resistor.However, I have not been able to get clean enough DC from the antenna to make the chip run. When the antenna, terminator, diode, and capacitor are connected to the breadboard, my scope shows that the chip's VCC pin is getting a sine wave, not clean DC power.What is the best way to rectify and filter RF into DC?1) Is a Schottky diode the best choice for rectification?2) Would a full wave rectifier using four diodes work any better than a single diode in this application?3) I have a toroid from a computer power supplies. It was designed to work as two parallel inductors to block RF. Would it help to add one to my circuit?4) Is there some other approach to filtering and rectification that would work even better?5) Is there any kind of transformer that would help to match the impedance, increase the current, or increase the voltage? Do transformers even work at 146 MHz?If I can get the circuit to work, I will demonstrate it at RANV HAM-CON in Vermont on February 25th. Visitors would be able to key up the handheld radio to make the circuit operate.

author: Ammar
author_rank: Regular Contributor
timestamp: February 04, 2017, 06:04:38 am
content: Interesting. My hunch is that the issue is your schottky diode not switching fast enough. When you load the circuit with 47ohms, the DC looks fine. This makes sense, since the load is significant enough to cause the diode capacitance to charge and discharge quickly enough to switch quickly enough to rectify the incoming signal.When you remove the 47ohm load and replace it with the 555 timer, the load may be so insignificant as to not allow the diode to switch fast enough.What diodes are you using? I have been using HSMS28XX series diodes which come in sot-23 packages for rectenna purposes. However, you may be able to get away with something cheaper at your operating frequency.Or, as suggested, you could piss away power into your shunt resistor. I hope that helps!

author: hobbyist
author_rank: Newbie
timestamp: February 04, 2017, 08:52:12 pm
content: The Schottky diode that I am using is an NTE583. Its packaging says "SI, Schottky, Fast Switching for UHF/VHF detection, 15 mA Max".Perhaps the problem comes from letting everything spread out over a breadboard. I will try to solder a better circuit to get shorter lead lengths.Here is what I will build, unless I get better advice first:BNC connector47 ohm terminator4 Schottky diodes as a full wave rectifier1 uF electrolytic capacitorToroid from a computer power supply.01 uF ceramic capacitor100 pF ceramic capacitorLMC555resistors and capacitors to make the LMC555 oscillate

author: Benta
author_rank: Super Contributor
timestamp: February 04, 2017, 09:08:04 pm
content: Interesting approach.Instead of saying "I want to achieve this and this, let me analyze the design and find the correct parts," you say: "I want to use an LMC555 and twist it to whatever the outcome may be."Very alternative...

author: Zero999
author_rank: Super Contributor
timestamp: February 05, 2017, 12:21:26 am
content: Perhaps you're going about it the wrong way?AM modulate the 145MHz signal with a 500Hz sine wave. Build a simple crystal radio with a Schottky diode and LC resonator. You might be able to increase the sensitivity by using two diodes to form a voltage doubler circuit.

author: hobbyist
author_rank: Newbie
timestamp: February 05, 2017, 04:50:56 am
content: Hero, you have a point. If I transmit modulated AM, then the antenna powered device could be a simple crystal radio. However,1) I don't want to demonstrate crystal radio again, although it went over well last time.2) My demo will be close to a 100 watt station that will be transmitting single sideband in the same HF bands where I would be transmitting AM. The radio would be overwhelmed by the stronger transmitter.Benta is right. I -do- want to twist things around until I can get DC from radio waves.1) Can anyone please point me to an existing circuit that converts RF into DC?2) I just tried the "Coilcraft RF Choke Finder", and it lists a tiny 1812CS-562 inductor with 61000 ohms of impedance at 146 MHz. Should I buy one and put it in front of the 555?