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Playback glitch on message recorder

We have purchased and built the Enhanced 45-Second Voice Recorder Module described in SILICON CHIP, December 2007. It works fine with the exception of one small but quite problematic glitch. When we record a message (we use tape mode for one full-length message) and we try to play it back, the Play function will not start until we have removed the 12V power to the unit for about 10-15 seconds. Then it plays back perfectly.

This happens each time you try to play the message back, by pressing the M1 message playback key.

We have looked at everything carefully but can not see why this is happening other than a capacitor is holding something at a certain potential that prevents the playback starting. When you remove the power to the unit, the capacitor will discharge and playback work again.

Do you have any suggestions as to how to fix this issue? We need to be able to continuously play the recorded message and since there is no loop function in the design, we need to trigger the playback M1 button each time the message has finished.

A loop or repeat function would be even better but we have now built a circuit to activate the playback button automatically. With the current issue unresolved, this won’t work of course. (J. D., via email).

• The manufacturer’s data on the HK828 voice recorder chip is a little ambiguous with regard to its operation in single message or “tape mode” and the chip does seem to behave differently in this mode (ie, when LK2 and LK3 are both in, pulling down both pins 24 & 25 to ground).

Basically, in this mode, the chip seems to have a choice of “normal” and “auto rewind” options, selected by either pulling pin 9 (M8 Enable-bar) high or low. But the data sheet is ambiguous regarding which is which, ie, whether pulling pin 9 high (as it is in our circuit) or pulling it down to ground instead gives auto-rewind operation.

We suggest that you try pulling pin 9 down to ground, to see if this will achieve auto-rewind operation (which should allow replay of the recorded message each time pin 1 is pulled briefly to ground).

By the way, according to the chip data, when it is operating in tape mode and with the auto-rewind option set correctly, it will loop around and play the message over and over if the M1-bar line (pin 1) is held down to ground. However there is a small pause of 1530ms of silence between the end of one replay and the beginning of the next.

Positive Line Switching With The Speed Controller

I recently bought a 10A/12-24V DC Motor Speed Controller (SILICON CHIP, June 1997) to run a wiper motor for a car. The car was changed from 6V to 12V and needed a suitable speed controller.

The problem is that the kit pulses on the negative line and not the positive. I cannot control the negative side of the motor as it is earthed through the body of the car.

I have been told that the control unit can be modified to pulse through the positive. Would you please be able to tell me how this is done? (P. V., via email).

• For positive line switching, the two Mosfets would need to be changed to P-channel types. A readily available P-channel Mosfet is the IRF9540N which has a rating of 100V and 23A (Jaycar ZT-2467). Possibly two of these in parallel would be suitable for a wiper motor.

They connect with the gates to the same 4.7Ω resistors in the circuit but the source (S) connects to the +12V supply (cathode of D1) and the drain (D) drives the positive side of the motor with the motor negative to ground.

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