We've all done it - turned on the oven to start it preheating and gone to do something else, only to come back much later after it's all done preheating and has been running for a half hour. Wouldn't it be nice if it beeped loudly when it's ready to cook? Some ovens do this already, but many don't, especially toaster ovens and pizza ovens. I designed this simple circuit to do just that, and it uses all discrete components so you can knowexactly how it works. Enhancement mode MOSFETs have been used to improve efficiency; so it only takes a small trickle to keep it operating.
The alarm is plugged into the wall and the oven is then plugged into the alarm in a pass-through type configuration. When the oven is turned on, the large heater(s) will draw a high current. The primary of X1 is connected in series with the heater and so this current flows through the primary also. This power is then transferred to the secondary by transformer action. The secondary has many more turns than the primary; so the high current and low voltage is converted into a low current and high voltage. Most preferably, X1 will be of the torroidal type and the primary will just go straight through.
The electrical "signal" coming from X1 is then rectified by a diode bridge and filtered by C1. C1 is there to prevent a wavering voltage which may cause the buzzer to be activated too soon. Z1, a 16V Zener, is connected across C1 to limit the voltage to a safe operating level for the gates of the 4 MOSFET transistors. Another capacitor, C2, is connected through diode D1 to effectively split the circuit into two parts: the first part (connected to the + of C1) which keeps the buzzer off while the heater is on, and the second part (connected to the + of C2) which powers the buzzer. D1 prevents the charge on C2 from flowing into the upper part of the circuit when the heater is off.
While the oven's heater is on, C1 remains charged which activates Q1, clamping the gate of Q2 to ground. Eventually, the thermostat will shut off the heater and cut power to the circuit. At this point, C1 will discharge quickly through R1. Since there is now no voltage to the gate of Q1, it will then shut off, releasing the clamp on Q2's gate. C2 cannot discharge through this path because of D1 and thus remains charged. With the clamping removed, Q2 is now turned on by the bias from R2 and begins simultaneously operating the buzzer and charging C3 through D4. Eventually C2 is discharged by the buzzer, leaving a charge on C3. (For a detailed description of the buzzer section, see my Piezo Buzzer circuit)
A charge on C3 activates Q3 which also clamps the gate of Q2. In order to prevent this from immediately shutting off Q2 and the buzzer, R3 and D3 are provided. D3 allows Q2 to give itself a very strong bias, and R3 weakens the clamping action of Q3 and also prevents C2 from quickly discharging through D3 and Q3. All of this works together to give us more "buzz time".
The heater in the oven will at some point be switched back on by the thermostat. When that happens, C1 and C2 will again charge as before and thus Q1 will also turn on to again clamp the gate of Q2. Again, the heater will be switched off once the oven gets back up to temperature. However, when Q1 turns off as before, Q2 will not turn on. This is because Q3 is still clamping the gate of Q2 due to the charge on C3. C3 will slowly bleed through R4 so that when the operator turns the oven off, after a wile the circuit will be as it was in the beginning with no charge on C3. A reset switch may also be provided to quickly drain the charge on C3.
Q4 is also activated by C3 in order to keep C3 charged while the oven is cycling on and off. D2 prevents C3 from discharging through the parasitic diode of Q4.
One drawback of the circuit is that if the heater is switched off and back on before C2 has a chance to discharge, Q2 will continue to bias itself into conduction and the buzzer will continue sounding until the heater is switched off. On the up side, this will tell you if your thermostat is on the fritz.
The alarm may be miniaturized so that it will fit entirely within the pass-through "plug" for a really compact little unit. Probably though, it will take more the form of a small project box with a cord and "plug" coming out one end, and another cord and "outlet" going out the other. Inside, one of the wires will pass through the core of X1 on its way through to form the primary. You can use the neutral wire, but it's probably better to use the hot wire. Otherwise, on ovens with a separate ground, ground loop currents might activate the alarm if you have faulty wiring in your house. This might be a good thing, but it will have you very frustratedwhile because the preheat alarm you just made will seem to be malfunctioning even though all of it was made right. At least it would until you figure out about the faulty wiring.