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<h2>555 Contest Entry</h2>
<span class="post-date" title="Date">February 23, 2011</span>
<span class="post-time" title="Time">10:19 pm</span>
<span class="post-author" title="Author">eric</span>
<span class="post-cath" title="Category"><a href="" title="View all posts in Projects" rel="category">Projects</a></span>
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<span class="entry"><div class="simplesocialbar-horizontal">
</div><p>Yes, it’s not really vacuum tube related, but I built an entry for the <a href="">555 timer contest</a>. It uses an <a href="">ICM7555</a>,
which is Maxim’s second source of Intersil’s CMOS version of Signetic’s
original NE555 timer. Turns out the fact that it is CMOS is important
for this particular circuit…</p>
<p><a title="ICM7555 - CMOS Timer IC by eschlaep, on Flickr" href=""><img src="Tube%20Time%20%C2%BB%20Blog%20Archive%20%C2%BB%20555%20Contest%20Entry_files/5472285745_f2b8d93c34.jpg" alt="ICM7555 - CMOS Timer IC" height="333" width="500"></a></p>
<p>My entry is an AM radio. The only active device (silicon, germanium,
or otherwise) is the ICM7555. The tuning is accomplished with an
inductor and a capacitor, and the ICM7555 acts as an AM demodulator and
class-D power amplifier to drive the speaker.</p>
<p><a href="" title="555 Radio by eschlaep, on Flickr"><img src="Tube%20Time%20%C2%BB%20Blog%20Archive%20%C2%BB%20555%20Contest%20Entry_files/5472285961_394fae970c.jpg" alt="555 Radio" height="333" width="500"></a></p>
<p>You may be wondering how all this is accomplished with a 555. The schematic is below.</p>
<p><a href="" title="555 Radio - Schematic by eschlaep, on Flickr"><img src="Tube%20Time%20%C2%BB%20Blog%20Archive%20%C2%BB%20555%20Contest%20Entry_files/5472389719_61b0b89372.jpg" alt="555 Radio - Schematic" height="310" width="500"></a></p>
<p>Here’s how the circuit works: The AM radio signal is tuned by
inductor L, which is 300 turns of wire on a 1/2 inch diameter cardboard
tube made out of an old toilet paper roll, along with the 100pF variable
capacitor. One end of the parallel configuration of L and C connects to
an antenna (surprisingly long!) and the other end connects to a ground
wire which is tied to the AC outlet ground (old books tell you to ground
it to a water pipe). So far this is exactly like an AM crystal radio.</p>
<p>The 555 timer is configured as a pulse width modulator in a
non-traditional configuration. If I used the standard approach and
connected the input to the CV pin, the low impedance of the pin would
prevent the circuit from receiving any radio signals. I had to invert
the circuit and tie both high impedance analog pins, Threshold and
Trigger to the radio signal input. This is the reason why the CMOS
version of the 555 timer performs much better than the standard bipolar,
which has higher input bias current. </p>
<p>The pulse width modulator ramp is created by the 0.01uF capacitor and
the 10K bias potentiometer which are connected to the Discharge pin.
The potentiometer wiper goes to the LC arrangement. With no radio signal
coming in, the voltage on Threshold/Trigger ramps up until it hits the
threshold, and then Discharge causes the voltage to ramp down again.</p>
<p>When a radio signal comes in, it gets superimposed on the ramp
signal, causing the threshold and trigger comparators to trip early or
late in a cycle. This variation causes the output duty cycle to vary,
which we can hear as sound in the speaker.</p>
<p>Demodulating the signal properly requires adjustment of the bias
knob, so that part of the radio signal is “clipped” and ignored by
either the threshold or trigger comparators. This ensures that the
negative “halves” of the radio wave don’t cancel out the positive
“halves”. </p>
<p>Want to hear what it sounds like? Check out the video below:</p>
<p><div role="button" tabindex="0" title=";fs=1" style="background: url(&quot;chrome://flashblock/content/flash.png&quot;) no-repeat scroll center center transparent; min-width: 32px ! important; min-height: 32px ! important; width: 425px; height: 344px; border: 1px solid rgb(223, 223, 223); cursor: pointer; overflow: hidden; display: inline-block; visibility: visible ! important; -moz-box-sizing: border-box;" bgactive="url(chrome://flashblock/content/flashplay.png) no-repeat center" bginactive="url(chrome://flashblock/content/flash.png) no-repeat center"></div></p>
<p>And of course, I can’t end the post without a gratuitous shot of the ICM7555 in circuit.<br>
<a href="" title="555 Radio - The Core by eschlaep, on Flickr"><img src="Tube%20Time%20%C2%BB%20Blog%20Archive%20%C2%BB%20555%20Contest%20Entry_files/5472285879_aeae2bcd04.jpg" alt="555 Radio - The Core" height="333" width="500"></a></p>
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<a name="comments"></a><div class="commtitle">19 Responses</div>
<ol id="commentlist">
<li class="graybox" id="comment-11822">
<!-- Gravatar -->
<h3 class="comment-title">Very cool! :</h3>
<p class="comment-meta">
Date: <a href="#comment-11822">
February 25, 2011 @ 1:51 am </a>
<div class="comment-text"><p>Wow! I wish my own projects could look as neat and be as innovative as this one. Great work! Well executed!</p>
<p>/Anders, Sweden</p>
<li class="alt" id="comment-11825">
<!-- Gravatar -->
<h3 class="comment-title">SockThief :</h3>
<p class="comment-meta">
Date: <a href="#comment-11825">
February 25, 2011 @ 6:15 am </a>
<div class="comment-text"><p>Hey Eric,</p>
<p>really awesome project, not only unexpected use of a 555 (I would
never have thought of it, that’s for sure) but it’s an incredibly
beautiful construction. </p>
<p>Is it possible to request a follow-up blog post on this? You briefly
touched on the circuit design and the importance of the CMOS version of
the 555, but I’d love to read some more on it.</p>
<li class="" id="comment-11826">
<!-- Gravatar -->
<h3 class="comment-title">Olivier :</h3>
<p class="comment-meta">
Date: <a href="#comment-11826">
February 25, 2011 @ 6:52 am </a>
<div class="comment-text"><p>Hi Eric,</p>
<p>I think your design is incredibly awsome ! The small distorted sound is doing so well with the wood and wires design.<br>
It is very far away from 555 use. I have made many odd things with it but not what you did.<br>
It is so bluffing that I hope you will win the contest.</p>
<p>Kind regards.</p>
<li class="alt" id="comment-11830">
<!-- Gravatar -->
<h3 class="comment-title"><a href="" rel="external nofollow" class="url">Scott Harden</a> :</h3>
<p class="comment-meta">
Date: <a href="#comment-11830">
February 25, 2011 @ 8:01 pm </a>
<div class="comment-text"><p>Very clever! I’m impressed! As a note to
newcomers, be very wary about thinking of AC ground as “near to earth
ground”. In the RF a good ground is just as important as your antenna,
and who knows what else AC ground is used for! Lights, other
electronics, air conditioners – all sorts of things tie into the same
wires which may push noise back into your radio. Living on the 2nd floor
of an apartment, my AC ground is noisy as can be! I have a radial
system of balanced dipole antennas hanging on my ceiling inside my
apartment to try to avoid using AC ground as ground. Also, that variable
capacitor is expensive and a little hard to find (I know!) – a
configuration with a varactor diode and potentiometer would be cheaper,
easier to access for anyone to recreate, and allow voltage controlled
(microcontroller?) tuning. I use power rectifier diodes successfully for
this purpose! Food for thought… but again, awesome project. You really
nailed the heart of the competition!</p>
<li class="" id="comment-11843">
<!-- Gravatar -->
<h3 class="comment-title">eric :</h3>
<p class="comment-meta">
Date: <a href="#comment-11843">
February 26, 2011 @ 3:04 pm </a>
<div class="comment-text"><p>Thanks Scott, you are right about the AC
ground. The tuning capacitor was in my junk box. Any cheap variable
capacitor would also work.</p>
<li class="alt" id="comment-12151">
<!-- Gravatar -->
<h3 class="comment-title">Norbert :</h3>
<p class="comment-meta">
Date: <a href="#comment-12151">
March 29, 2011 @ 4:26 pm </a>
<div class="comment-text"><p>Hi Eric!<br>
The construction is very super, but your schematic diagram is wrong. (If I good think, this content two errors.)<br>
1., The treshold (pin6) not comon the trigger (pin2). The trigger
connect the antenne and paralel resonant circuit. The treshold connect
the timer tags. If not connect, then this circuit timing is nothing.<br>
2., The timer 0.01uF you connect VCC and GND. The cap is charged _once_ and…? <img src="Tube%20Time%20%C2%BB%20Blog%20Archive%20%C2%BB%20555%20Contest%20Entry_files/icon_smile.gif" alt=":)" class="wp-smiley"><br>
A think, the cap and potmeter serial connection. Common point the
cap&amp;pot connect the discharge (pin7) and potmeter mid point connect
the treshold (pin6)? My friend testing tomorrow this modifications.<br>
If my idea is good, then please correct the scheme. Thx., best regards: Norbert Kis (Hungary)</p>
<li class="" id="comment-12155">
<!-- Gravatar -->
<h3 class="comment-title">JD :</h3>
<p class="comment-meta">
Date: <a href="#comment-12155">
March 30, 2011 @ 1:31 am </a>
<div class="comment-text"><p>The circuir, construction, shamless plug
for Maxim, and VIDEO, are all AWESOME. I love the almost subconcious
snipits in the video. How was it produced. Who is the producer ?
Anyway … You have a nonlinear ramp, your audio will distort at any
volume. I suggest you impliment a Threshold Optibias current source,
then check the noise performance against a differential reference.
Anyway …<br>
Thinking outstde the box to solve practical issues gets Eric my vote as
#1 555 application. Even the inventor of the 555 topology and original
IC was impressed with this innovation,<br>
How many uH is the L, range of C =&gt; 1/2*pi()*sqrt(L*C) =&gt; Tuning range.</p>
<p>This appears to be a direct conversion topology, unless the pointer
on the pot is really a germanium diode. Where is the signal detected ?</p>
<li class="alt" id="comment-12349">
<!-- Gravatar -->
<h3 class="comment-title">mukesh harsh :</h3>
<p class="comment-meta">
Date: <a href="#comment-12349">
April 21, 2011 @ 11:13 am </a>
<div class="comment-text"><p>is there possible to carry video signal in such circuit?</p>
<li class="" id="comment-12351">
<!-- Gravatar -->
<h3 class="comment-title">Dave B :</h3>
<p class="comment-meta">
Date: <a href="#comment-12351">
April 21, 2011 @ 12:28 pm </a>
<div class="comment-text"><p>@Norbert:<br>
I thought the same as you did, but just realized my error. The .01uF cap
is sequentially charged and discharged through the pot, so long as the
output is switching between Hi and Low, which alternates driving pin 7
(Dis) to battery return, pin 1. The ground on the other side of the cap
is not tied to battery return, but to Earth ground AND to the low side
of L, therefore (at DC or Audio frequencies) to Trig and Thresh!</p>
<p>VERY clever design!</p>
<li class="alt" id="comment-13197">
<!-- Gravatar -->
<h3 class="comment-title">shane maxfield :</h3>
<p class="comment-meta">
Date: <a href="#comment-13197">
June 22, 2011 @ 7:20 pm </a>
<div class="comment-text"><p>10 years ago i made this 555 radio.</p>
<li class="" id="comment-13685">
<!-- Gravatar -->
<h3 class="comment-title">Steve :</h3>
<p class="comment-meta">
Date: <a href="#comment-13685">
July 24, 2011 @ 9:13 pm </a>
<div class="comment-text"><p>It seems that this is operating at about
33 MHz to 50 MHz maybe. 100pF and the 1/2 inch air core inductor of 300
turns comes in about 230nH maybe. That would be Fire, Government, Police
band kind of stuff.</p>
<p>What did you hear?</p>
<li class="alt" id="comment-14674">
<!-- Gravatar -->
<h3 class="comment-title">Rick Herndon :</h3>
<p class="comment-meta">
Date: <a href="#comment-14674">
October 4, 2011 @ 3:50 pm </a>
<div class="comment-text"><p>A ham radio friend commented quite completely on this project. He gave permission to forward it here:<br>
Using a MAXIM ICM7555 as an AM broadcast band receiver is innovative indeed!</p>
<p>The amount of ICM7555 device capacitance that adds to the 100 pF air-variable tuning capacitor is<br>
negligible, and the antenna-to-earth capacitance also is negligible. Ignoring these negligible<br>
capacitances, the resonate frequency exhibited by the 100 pF air variable tuning capacitor and<br>
the inductor is what determines the tuning of the AM receiver.</p>
<p>I believe the inductor is poorly dimensioned. From the description and the picture, I estimate<br>
that the inductor has 300 turns of #33 enamel-coated wire, in a single-layer solenoid form of<br>
0.5071 inch wire-center-to-wire-center inductor diameter, close-wound over a 2.367 inch length).<br>
As such, the inductor has an inductance of about 223µH. With the variable capacitor set to<br>
full-mesh at 100pF, the resonant frequency is 1066 kHz and this is the low frequency limit of the<br>
tuning range. Assuming the unmeshed value of the variable capacitor is 9.6pF, then the high<br>
frequency limit of the tuning range is 3440 kHz (reaches the 87 meter band). Aside from<br>
exhibiting insufficient inductance for the tuning range of the AM broadcast band, the inductor<br>
has low Q. The description of the inductor, “300 turns of wire on a 1/2 inch diameter cardboard<br>
tube made out of an old toilet paper roll”, constrains the Q to be low because the excessive<br>
ratio of length to diameter of the inductor (about 4.668:1) precludes the obtainment of high Q.<br>
What to do, what to do?</p>
<p>I suggest replacing the original inductor with an alternate inductor that uses an ordinary 1.75<br>
inch diameter toilet paper roll as its form. The increase of form diameter, from 0.5 inch to<br>
1.75 inch, is beneficial for obtaining greater inductance and for obtaining greater Q. Instead<br>
of #33 enamel-coated wire as used in the original inductor, an alternate inductor made of #28<br>
enamel-coated wire or an alternate inductor made of #26 enamel-coated wire is suggested. See<br>
table below for the inductor specifications, and compare the original inductor with the two<br>
suggested alternate inductors: Alternate 1 and Alternate 2. Unlike the original inductor, both<br>
of the alternative inductors provide essentially complete tuning range coverage of the AM<br>
broadcast band.</p>
| Inductor specifications | Original | Alternate 1 | Alternate 2 |<br>
| Wire gauge | #33 | #28 | #26 |<br>
| Wire total length (inches) | 478 | 1124 | 1309 |<br>
| Wire thickness (inches) | 0.0071 | 0.0126 | 0.0159 |<br>
| Form diameter (inches) | 0.50 | 1.75 | 1.75 |<br>
| Inductor diameter (inches) | 0.5071 | 1.7626 | 1.7659 |<br>
| Inductor length (inches) | 2.367 | 2.761904762 | 4.013605442 |<br>
| Inductor length/diameter | 4.667718399 | 1.566949258 | 2.272838463 |<br>
| Numer of turns | 300 | 203 | 236 |<br>
| Inductance (µH) | 222.9460301 | 900.3075203 | 903.0426128 |<br>
| Resonance with 100pF (kHz) | 1065.909321 | 530.4258643 | 529.6219909 |<br>
| Resonance with 9.6pF (kHz) | 3440.207540 | 1711.942116 | 1709.347624 |<br>
<p>Note that the “Inductor diameter” specification includes the “Wire thickness” in addition to the<br>
“Form diameter”. In other words, “Inductor diameter” amounts to the wire-center-to-wire-center<br>
<p>73 to K5FNI de WD8PNL, -= Ron Wiesen =-<br>
Note: in order to read the table, you may have to block and copy it out to a wider display than is possible here.</p>
<li class="" id="comment-14675">
<!-- Gravatar -->
<h3 class="comment-title">Rick Herndon :</h3>
<p class="comment-meta">
Date: <a href="#comment-14675">
October 4, 2011 @ 3:51 pm </a>
<div class="comment-text"><p>Ahh, I see that the font here is not
monospaced like Courier. If you want to see the table above, you will
have to block it and print it out in a monospaced font like Courier.</p>
<li class="alt" id="comment-15599">
<!-- Gravatar -->
<h3 class="comment-title">Alex :</h3>
<p class="comment-meta">
Date: <a href="#comment-15599">
December 27, 2011 @ 10:27 am </a>
<div class="comment-text"><p>Ok, it’s cool but if this device works
only with external antenna and ground – i prefer to use the 1zh29b tube
as a grid detector o regenerative receiver with 9V (plate) and 1.2V
(heater). Best regards to all the HAM’s! 73!</p>
<li class="" id="comment-15692">
<!-- Gravatar -->
<h3 class="comment-title">Billy :</h3>
<p class="comment-meta">
Date: <a href="#comment-15692">
January 1, 2012 @ 9:55 am </a>
<div class="comment-text"><p>This project seems like exactly what I
was hoping for. A simple project to use as a starting point with radio. I
have three questions to help me get started.<br>
1) Would it be possible to make a transmitter with a similar circuit?<br>
2) Could a typical 555 work instead of the 7555?<br>
3) Would you consider posting this to instructables with step by step instructions?</p>
<p>Also: I reformatted Rick’s table in notepad with tabs and I’m also
posting it in code tags since I think they might use monospace fonts.
Lets hope it works!</p>
| Inductor specifications | Original | Alternate 1 | Alternate 2 |<br>
| Wire gauge | #33 | #28 | #26 |<br>
| Wire total length (inches) | 478 | 1124 | 1309 |<br>
| Wire thickness (inches) | 0.0071 | 0.0126 | 0.0159 |<br>
| Form diameter (inches) | 0.50 | 1.75 | 1.75 |<br>
| Inductor diameter (inches) | 0.5071 | 1.7626 | 1.7659 |<br>
| Inductor length (inches) | 2.367 | 2.761904762 | 4.013605442 |<br>
| Inductor length/diameter | 4.667718399 | 1.566949258 | 2.272838463 |<br>
| Numer of turns | 300 | 203 | 236 |<br>
| Inductance (µH) | 222.9460301 | 900.3075203 | 903.0426128 |<br>
| Resonance with 100pF (kHz) | 1065.909321 | 530.4258643 | 529.6219909 |<br>
| Resonance with 9.6pF (kHz) | 3440.207540 | 1711.942116 | 1709.347624 |<br>
<li class="alt" id="comment-15693">
<!-- Gravatar -->
<h3 class="comment-title">Billy :</h3>
<p class="comment-meta">
Date: <a href="#comment-15693">
January 1, 2012 @ 9:57 am </a>
<div class="comment-text"><p>Apparently my attempt at fixing the table didn’t work. Oh well.</p>
<li class="" id="comment-16241">
<!-- Gravatar -->
<h3 class="comment-title">eric :</h3>
<p class="comment-meta">
Date: <a href="#comment-16241">
January 22, 2012 @ 11:36 pm </a>
<div class="comment-text"><p>Here are some answers to your questions:</p>
<p>1) Maybe. You would need a circuit that oscillates and modulates.
Since the 555 is an oscillator, you’re already on the right track.<br>
2) The 7555 is a CMOS version of the bipolar 555. The most noticeable
difference is that the 7555 has high impedance inputs while the 555 does
not. My radio circuit needs the high impedance to function, so the 555
would not work in it.<br>
3) Hadn’t thought of the Instructables thing. There are some very good
beginning radio websites out there that do a good job explaining how to
build simple radios, so I am not sure what I could really contribute
with my “hack”.</p>
<p>The table didn’t come out, unfortunately. I’ve been meaning to try
out a different coil design, but there are so many new projects I’ve
been working on…</p>
<li class="alt" id="comment-17306">
<!-- Gravatar -->
<h3 class="comment-title">Eugen Grzondziel :</h3>
<p class="comment-meta">
Date: <a href="#comment-17306">
March 12, 2012 @ 3:27 am </a>
<div class="comment-text"><p>A very brilliant idea, but does it work
really as described? (as D-class amplifier) i am not sure. The circuit
works as radio WITHOUT capacitor 10 nF too.</p>
<li class="" id="comment-17967">
<!-- Gravatar -->
<h3 class="comment-title">eric :</h3>
<p class="comment-meta">
Date: <a href="#comment-17967">
April 4, 2012 @ 10:15 am </a>
<div class="comment-text"><p><cite>old books tell you to ground it to a water pipe</cite><br>
In my house, AC ground goes right to a copper water pipe in the basement.</p>
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