Vibrating VU meter

This circuit was designed to enable a visually impaired person to work as a presenter at the local community radio station. It vibrates when the signal level in either of the stereo channels exceeds 0VU. Even if you aren’t visually impaired, it’s a handy device to monitor recording and broadcast levels, as it frees you from watching level meters or overload lights.

The circuit is designed to fit into the case of a surplus pocket pager. This makes it comfortable to hold, put in a pocket, or clip to a belt. The pager also provides the vibrating motor and an easy-to-access battery compartment for a single AAA cell (or AA cell in some models).

The circuit design was constrained by the supply voltage of only 1.5V, virtually forcing the use of discrete transistors.

The left and right channel signals cannot be simply added together, because a negative swing on one channel could then mask an overloading positive swing on the other channel. Hence separate transistors, Q1 and Q2, are used to detect overload in each channel. The collectors of Q1 and Q2 are connected together, wired-OR fashion, to give a signal that goes low when there is an overload in either channel.

The signal from each input channel is attenuated by a 100kW trimpot, adjusted so that a 0VU signal will just turn on the appropriate transistor. The threshold is referenced only to the turn-on voltage of the transistor, making it independent of the supply voltage.

Transistor Q3 provides extra gain to square up the response to input signals near the threshold. The 1μF capacitor and parallel 470kΩ resistor extend the response to brief overloads to a minimum of about 1/3 second. Transistors Q4 and Q5 form a Schmitt trigger which ensures clean transitions as the 1μF capacitor charges and discharges. Transistors Q6 and Q7 act as current amplifiers to drive the pager motor, which needs about 60mA for reliable starting.

Switch S1 and the series 1M resistor provide an optional battery test facility. Pressing S1 will activate the pager motor, provided the battery voltage exceeds about 1.2V.

There is no power switch because all transistors are off when the input signal is below the trigger threshold, giving negligible quiescent current drain.

Andrew Partridge, Kuranda, Qld.

Digital Trainer using 4000 series chips This circuit is a simple cheap way of teaching the basics of digital electronics and the functions of different types of gate. The basic circuit is the same for all gate types; one gate has its inputs initially tied low. Two switches allow you to change the level on an input and therefore change the output which drives a LED to show the output state; on is High. You can use this circuit to demonstrate the functions of AND (4081), OR (4071), NOR (4001), Exclusive-OR (4030, 4070) and Exclusive-NOR (4077) gates. The use of a socket allows convenient changing between chips. Only the first gate of the chip is used, with all the other inputs being tied to ground (0V) via a 100kΩ resistor. The circuit will run on supplies between 5V and 12V DC. Owen Klan, Rathdowney, Qld. ($40)

Thermistor addition for heat controller

This circuit is essentially the Heat Controller published in the July 1998 issue of SILICON CHIP with the added feature of thermistor control.

As originally published, the Heat Controller circuit used a 1MW potentiometer to control the power delivered to the load. This circuit modification employs comparator IC3 and a thermistor to provide temperature control and gates the Triac control signal on and off with IC2, a 4001 quad NOR gate.

When the temperature is lower than the setting desired by trimpot VR2, the thermistor’s (relatively) high resistance will mean that pin 2 of IC3 will be high and so pin 1 will be low. This signal is fed to three gates of IC2 so that the pulse signal from IC1 is fed through to IC4 and the Triac to drive the heater load.

Once the temperature rises sufficiently, the thermistor’s resistance will drop, pulling pin 2 of IC3 below pin 3 and sending pin 1 high. This will cause the NOR gates of IC2 to block of signals and so the Triac will not be pulsed on and off.

Jeff Griffin, Bairnsdale, Vic. ($40)