Justin Olexy (jut@netaxs.com) penned:
: Lo all... I'm fixing up my project for the Duracell electronics
: competition, and I need a bit of help with an improvement I am making. I
: need to be able to trigger a relay (or other near-zero resistance switch)
: from the audio input of a microphone or small speaker. I have made a
: 2-transistor (NPN) amplifier which helps, but the audio into the
: microphone (using small speaker) has to be very loud (actually, I have to
: tap on the speaker to get the relay to trigger). I need some way to
: either boost the audio input, or another way to connect a circuit using
: audio. Any ideas? Thanks
Speakers work OK as microphones, but they're low impedance. If you put a
transformer between the speaker and amp, then the impdeance and voltage
will be higher. The transformer from Rat Snack, with a 1K primary and 8
ohm secondary will work. But the 8 ohm secondary is connected to the
speaker and the primary to the amp. This might not increase the voltage
enough, since it seems from your description that you have an amp with
very poor sensitivity. You may need another stage of amplification ahead
of it.
Don't worry. A single transistor with the emitter grounded, a 10k
resistor from the collecctor to the positive supply.. Oh, yeah, I forgot
that most people don't understand tech talk. Here.
----
Basic Single Stage Amplifier
A Schematic Ver. 951229a
Legend: ) = No Connection
+ = Connection
All resistors are 1/4 W 5%.
All capacitors are 16V or more.
J1 Audio Input
+--------------------( O )
| |
| |
| |
| ----- C1 .05 to .1 uF
| -----
| R2 47K | + R1 470K
+-------/\/\/\---------+-------------/\/\/\------------+
| | |
| | |
| | |
| | Q1 NPN |
| | 2N3904 |
| ------- |
| R4 470 E / \ C R3 10K |
+------/\/\/\-----/ \-----+-----/\/\/\---------+
| | |
| | |
| | + |
| ----- C2 .1 uF |
| ----- |
| | |
| | |
| R5 50K | |
+---------------/\/\/\/\/\--------+ +
| /|\ C3 47 uF |
| | || + |
+--------------------)------------------||-------------+
| | || |
| | |
+------------------( O ) |
| J2 Audio Output |
| - 9VDC + |
C1 .05 uf to .1 uF 50 or more volts
C2 0.1 uF 50V or more
Q1 2N3904 NPN low noise audio transistor
J1, J2 Jack to accomodate the equipment and cables
R1 470K 5% 1/4W
R2 47K 5% 1/4W
R3 10K 5% 1/4W
R4 470 ohm 5% 1/4W
R5 50K Audio Taper Potentiometer
The unbypassed R4 emitter resistor causes the imput impedeance to be
high, and it causes a loss of gain. The amplifier stage cannot have a
gain greater than the ratio of R3 to R4, so 10000/470 is about 20.
The coupling capacitors C1 and C2 are fairly small, about 1/10 uF.
This reduces the gain at low frequencies. The capacitors can be
larger for Hi Fi use. Anywhere from 1 uF to 10 uF should be OK.
The input impedance is several tens of Kohms. The output impedance
depends somewhat on the setting or the R5 level pot. It can't be
higher than 15 K, but it can be as low as the pot setting. The DC
supply should be well filtered to keep any hum and noise out of the
audio. If a lower impedance is needed to drive long lengths of cable
or a pair of earphones, then an emitter follower stage should be added
to increase the output current.
With a maximum peak-to-peak voltage at the collector of approx 8V, the
RMS output voltage should be about 2.5 to 2.8 VRMS maximum.
Total circuit current drain on the battery is less than a half
milliamp, probably around 400 uA.
----
: --
: jut <> Justin Olexy <> "http://www.voicenet.com/~jut1"
--
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Date: 10 Feb 1996 19:15:13 GMT
Original Subject: Re: Audio relay trigger info needed