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Per Colin you need to provide the part numbers and component values. At a glance the main chip with the LEDs connected is likely a 4017 decade counter divider. When the microphone picks up a loud enough sound the counter will increment up one count so the LEDs will sequentially light and turn off. That is strictly a guess.
I found your circuit with the explanation. Just like the last circuit you posted.
What part of this do you not understand?
Device purpose:
The basic circuit illuminates up to ten LEDs in sequence, following the rhythm of music or speech picked-up by a small microphone. The expanded version can drive up to ten strips, formed by up to five LEDs each, at 9V supply.
Circuit operation:
IC1A amplifies about 100 times the audio signal picked-up by the microphone and drives IC1B acting as peak-voltage detector. Its output peaks are synchronous with the peaks of the input signal and clock IC2, a ring decade counter capable of driving up to ten LEDs in sequence.
An additional circuit allows the driving of up to ten strips, made up by five LEDs each (max.), at 9V supply. It is formed by a 10mA constant current source (Q1 & Q2) common to all LED strips and by a switching transistor (Q3), driving a strip obtained from 2 to 5 series-connected LEDs. Therefore one transistor and its Base resistor are required to drive each of the strips used.
Notes:
The sensitivity of the circuit can be varied changing R4 value.
C4 value can be varied from 220 to 470nF in order to change the circuit speed-response to music peaks.
Adopting the additional circuit, only one item for R10, R11, Q1 and Q2 is required to drive up to ten LED strips. On the contrary, one item of R9 and Q3 is necessary to drive each of the strips you decided to use.
Each R9 input must be connected to IC2 output pins, in place of the LEDs D2-D11 shown. R8 must also be omitted.
Wishing to use a lower number of LEDs or LED strips, pin #15 of IC2 must be disconnected from ground and connected to the first unused output pin.
For example: if you decided to use 5 LEDs, pin #15 of IC2 must be connected to pin #1; if you decided to use 8 LEDs, pin #15 of IC2 must be connected to pin #9 etc.
Current drawing of the circuit is about 10mA.
Wishing to use a wall-plug adapter instead of a 9V battery, you can supply the circuit at 12V, allowing the use of up to 6 LEDs per strip, or at 15V, allowing the use of up to 7 LEDs per strip.
This is the first circuit you posted which has a detailed explanation on the website you got it from as does the new circuit you posted. Just ask about what you do not understand.
Adopting the additional circuit, only one item for R10, R11, Q1 and Q2 is required to drive up to ten LED strips. On the contrary, one item of R9 and Q3 is necessary to drive each of the strips you decided to use.
I'm not sure why they suggest doing it that way. Maybe another member has a thought on it. If I wanted to drive a load exceeding what the 4017 can support I would just come off the 4017 to the base of a NPN like the one they used or a 2N2222 which can handle a forward current of 800 mA and use current limiting resistors to the rail. I see where Q1 will allow a drop of about .7 volts across R11 and Q2 will be conducting depending on R10 but fail to understand why they use them at all? While I can see where Q1 and Q2 limit the current based on Q2 conduction I don't see the merit to the added transistors rather than using a single resistor to limit the LED current right off the rail. Beats me as in I don't know. Again, maybe someone else sees the merit to it?
Unless since they mention: "D12,D13 etc.____5 or 3mm. LEDs (any type and color)" it allows for a wide range of LEDs to be used as they do not mention the LED forward voltage or current?
The current sourcing transistors allow the strings of LEDs to have different numbers of LEDs in series and different coloured LEDs (different forward voltages).
The current sourcing transistors allow the strings of LEDs to have different numbers of LEDs in series and different coloured LEDs (different forward voltages).
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