WHo can tell me what is wrong with this circuit?


Joined Dec 26, 2010
I would think that putting a tank coil straight to ground from the 555 output would cause far too much DC output current flow. The tank filtering will also be pretty much of an illusion driven from a totem-pole output.


There are others about with particular interest in 555 circuits who may comment, but I don't think this arrangement is any good.

Thread Starter


Joined Jan 30, 2011
It was the only thing I could see that would cause it to heat up so bad. I guess just lower the input voltage.... I used a 1000uh coil/200 pf cap.


Joined Dec 26, 2010
A DC blocking cap and a series resistor in the line to the tuned circuit would be more to the point, so that the poor old 555 gets a manageable load.

That said, this thing may be illegal anyhow. What frequency are you trying to send? (356kHz???)


Joined Oct 22, 2011
your pin 5 isnt conncted..
pin 5 should is frequently connected through a 0.01uf(10nF) cap.
the potentiometer isnt going to be doing much with the wiper and one end connected to supplyV

Thread Starter


Joined Jan 30, 2011
This is a 2 part circuit. This is the transmitter portion. I made the 1000uh inductor coil, and used a 200pf cap. The receiver portion will have the same 1000uh coil, with a 75-250pf trimmer. I used a 9V battery snap, but tried the specified 12v supply....Hot as hell with seconds.


Thread Starter


Joined Jan 30, 2011
Yes, comes out to 356 khz. This is a locating device. The other part of the circuit has 5 led's that will illuminate as you near the transmitter.


Joined Jul 9, 2011
I wonder if anybody ever built this and it was working? :rolleyes:

The 555 frequency is adjustable from 1.17kHz upwards. Then the LC at the output could be anything between 187kHz and 1.6MHz. :confused:
Every time the 555 output switches (with it's much lower output frequency) it would cause an oscillation in the LC which would immediatly cease IMO because the output will be either tied to + or GND. The rest of the time current will flow through the inductance when the 555 output is HIGH, causing it to heat up...


Joined Mar 24, 2008
Lowering the input voltage voltage will not help, the coil is a dead short. A 555 connects directly to ground, and couples to Vcc pretty tightly when it is switched that direction. You can isolate pin 3 via capacitor, or find some other way to do it. I assume you are trying to filter out a sine wave? A 3 stage low pass RC filter might do much better.

You could move C3 to pin 3.

I've seen a lot of circuits trying to use the 555 for an AM circuit. A 555 is basically a digital chip, this does not work very well I'm afraid. You would produce more power, use fewer components and produce a much cleaner signal with a simple transistor oscillator. The 555 is about as unsuitable for this as anything I've ever seen.


Joined Dec 20, 2007
The transmitter is a very poor design.

The receiver is also a joke. Its lousy old 741 opamp has trouble above 9kHz and here it is trying to amplify 356kHz. It should be a high frequewncy opamp with a high slew rate.
The tuned circuit is shorted by the very low 1k ohm input impedance of the inverting opamp circuit. It should be a non-inverting opamp then it can have a high input impedance so the tuned circuit is not loaded down and can have good selectivity.

Why do the LEDs have parallel resistors?
With the plus and minus 9V supply, the output of the 741 opamp will not go high enough to light 5 LEDs in series.
in such circumstances you must follow these steps:
1. check that the hot ic is working of the circuit or is burned (you can check using tester)
2. if ic is working then find all currents to and from the ic and compare it with specification if it is correct then come the then
3.you must recognize that you are using inductor in your circuit so two problems might be there:
1 peak inverse voltages are damaging the circuit or
2 there is a problem of interfacing you must draw block diagram of the circuit and determine the ports in between the blocks now determine the currents and power transmission i am sure that the problem lies in interfacing so you must add circuits or components require to compensate the effect.


Joined Oct 26, 2011
isolate pin 3 ,switch on and see if yr 555 gets hot.....decouple pin 5 to pin 1 with a 0.1 mf cap....also disconnect one end of yr preset(the end to pin 7...)and insert a 1k resistor there let me know if you have a working osc. with no heating 555