I'm making a wireless remote control system using the 555 and 567 IC's.

Everything is working OK with the exception of the tone detector, yet I can hear the tone when I hook up a speaker to my circuit at the point where the tone detector can pick the same noise up.

I think it comes down to the input filter capacitor and output filter capacitor connected to the 567.

Currently I use 2.2uF and 4.7uF capacitors as filter capacitors, but there is nowhere online that dictates an equation for such capacitors and there's nothing that tells me what resistance to follow so I can even figure out a capacitor value.

The tone I'm trying to detect is a tone at 5Khz.

What equation do I use for the input and output filter capacitor for the 567 to successfully detect a 5Khz tone?

  • \$\begingroup\$ Schematic? Link to datasheet - for the 567 at least? \$\endgroup\$
    – Transistor
    Commented Sep 10, 2016 at 6:36

2 Answers 2


Making it go:

It is likely that the lack of output is due to you not having allowed for the fact that the LM567 has an "open collector" output.
(Rl on pin 8 in the diagram below. Output is low when tone is 'detected'.)

It needs a resistor or similar to V+ to operate.
Without this the device is about as good at detecting tones as is a pillar of salt.


The Philips data sheet makes design easy [tm].

Philips LM567 data sheet
FAR better for design and understanding than TI version.
Page 408 contains most of what you need to know, but there is also much more there that is useful.

enter image description here


Frequency = f0 = 1/(1.1 x R1 x C1)
( 2k <= R1 <= 20k)

Bandwidth = 1070 x (Vi/(f0 x C2))^1/2
20 mV <= Vi < 200 mV - input voltage

Output filter capacitor C3 - From Philips data sheet page 409
"The value of C3 is generally non-critical. C3 sets the band edge of a low-pass filter which attenuates frequencies outside the detection band to eliminate spurious outputs. If C3 is too small, frequencies just outside the detection band will switch the output stage on and off at the beat frequency, or the output may pulse on and off during the turn-on transient. If C3 is too large, turn-on and turn-off of the output stage will be delayed until the voltage on C3 passes the threshold voltage. (Such delay may be desirable to avoid spurious outputs due to transient frequencies.) **A typical minimum value for C3 is 2 x C2

See Philips data sheet for other components.


TI LM567 data sheet here - Detailed design requirements are given in section on page 14.
Shockingly poor wrt applications.

Natsemi - looks familiar :-(.

Here is an online calculator for all tone related components

Features & parameters explained they say

Lots of ideas - each image links to a page

LM567 used in IR controller

A very old but useful IC - I used these in my Masters thesis hardware 35+ years ago :-).

  • \$\begingroup\$ +1 for making reference to C2 equation in the Philips LM567 data sheet \$\endgroup\$
    – user116345
    Commented Sep 10, 2016 at 20:05
  • 1
    \$\begingroup\$ +1 for the much better Philips datasheet. I just got bit by the TI/NatSemi datasheet and it's bad design guideline. Using an R1 < 2kOhm doesn't go well. \$\endgroup\$
    – remcycles
    Commented Jun 14, 2018 at 19:12

but there is nowhere online that dictates an equation for such capacitors and there's nothing that tells me what resistance to follow so I can even figure out a capacitor value

It's really easy to find this information: -

enter image description here

enter image description here

  • 1
    \$\begingroup\$ I can't accept this answer because I'm trying to figure out how to calculate values of C3 and C2. What you and other sources online have demonstrated is how to determine the detection frequency using C1 and R1. \$\endgroup\$
    – user116345
    Commented Sep 10, 2016 at 19:58

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