Crowbar Speaker Protection Circuit Diagram

Crowbar circuits are so-called because their operation is the equivalent of dropping a crowbar (large steel digging implement) across the terminals. It is only ever used as a last resort, and can only be used where the attached circuit is properly fused or incorporates other protective measures.

A crowbar circuit is potentially destructive - if the circuitry only has a minor fault, it will be a major fault by the time a crowbar has done its job. It is not uncommon for the crowbar circuit to be destroyed as well - the purpose is to protect the device(s) attached to the circuit - in this case, a loudspeaker.

Description

Theres really nothing to it. A resistor / capacitor circuit isolates the trigger circuit from normal AC signals. Should there be enough DC to activate the DIAC trigger, the cap is discharged into the gate of the TRIAC, which instantly turns on ... hard. A TRIAC has two basic states, on and off. The in-between state exists, but is so fast that it can be ignored for all intents and purposes.

 Figure 1 - Crowbar Speaker Protector

The BR100 DIAC (or the equivalent DB3 from ST Microelectronics) is rated for a breakdown voltage of between 28 and 36V - these are not precision devices. Needless to say, using the circuit with supply voltages less than around 40V is not recommended, as you will have a false sense of security. The supply voltage must be higher than the breakdown voltage of the DIAC, or it cannot conduct. Zeners cannot be used as a substitute for lower voltages - a DIAC has a negative impedance characteristic, so when it conducts, it will dump almost the full charge in C1 into the gate of the TRIAC. This is essential to make sure the TRIAC is switched into conduction.

The TRIAC is a common type, and may be substituted if you know the specifications. Its rated at 12A, but the peak current (non-repetitive) is 95A, and it only needs to sustain that until the fuse (or an output transistor) blows. A heatsink is preferred, but there is a good chance that the TRIAC will blow up if it has to protect your speakers, so it may not matter too much. The 0.47 ohm resistor is simply to ensure that the short circuit isnt absolute. This will limit the current a little, and increases the chance that the TRIAC will survive (albeit marginally). Feel free to use a BT139 if it makes you feel better - these are rated at 16A continuous, and 140A non-repetitive peak current.

The peak short circuit current will typically be about 90A for a ±60V supply, allowing ~0.2 ohms for wiring resistance and the intrinsic internal resistance of the TRIAC, plus the equivalent series resistance of the filter capacitors. Thats a seriously high current, and it will do an injury to anything thats part of the discharge path. Such high currents are not advised for filter caps either, but being non-repetitive they will almost certainly survive.

Construction & Use

Apart from the obvious requirement that you dont make any mistakes, construction is not critical. Wiring needs to be of a reasonable gauge, and should be tied down with cable ties or similar. C1 must be polyester. While a non-polarised electrolytic would seem to be acceptable, the circuit will operate if the capacitor should dry out over the years. This means it will lose capacitance, and at some point, the crowbar may operate on normal programme material. This would not be good, as it will blow up your amplifier!

Make sure that all connections are secure and well soldered. Remember that this is the last chance for your speakers, so it needs to be able to remain inactive for years and years - hopefully it will never happen. The circuit doesnt have to be mounted in the amplifier chassis - it can be installed in your speaker cabinet. Nothing gets hot unless it operates, at which point no-one really cares - it just has to save the speakers from destruction once to have been worthwhile.

Remember that the crowbar circuit absolutely must never be allowed to operate with any normal signal. A perfectly good amplifier that triggers the circuit because of a high-level bass signal (for example) will very likely be seriously damaged if the crowbar activates. To verify that no signal can trigger it, you may want to (temporarily) use a small lamp in place of R2, and drive the amp to maximum power with bass-heavy material.

A speaker does not need to be connected. If the lamp flashes, your amp would have been damaged. If this occurs, you may want to increase the value of C1. Note that bipolar electrolytics should never be used for C1, because they can dry out and lose capacitance as they age. This could cause the circuit to false-trigger.

Wire Loop Game Circuit Diagram

In the ‘Wire Loop Game’, a test of dexterity,  the player has to pass a metal hoop along a  twisted piece of wire without letting the hoop  touch the wire. Usually all the associated electronics does is ring a bell to indicate when the  player has lost. The version described here has  a few extra features to make things a bit more  exciting, adding a time limit to the game and a ticking sound during play. 

Two 555 timer ICs are used to provide these  functions. IC1 is configured as a monostable which controls the time allowed for the  game, adjustable using potentiometer P1. IC2  is a multivibrator to provide the ticking and Two 555 timer ICs are used to provide these  functions. IC1 is configured as a monostable which controls the time allowed for the  game, adjustable using potentiometer P1. IC2  is a multivibrator to provide the ticking and he continuous buzz that indicates when the  player has touched the wire with the hoop. 

Wire Loop Game Circuit diagram :

Wire Loop Game Circuit Diagram

When the monostable is in its steady state,  the output of IC1 (pin 3) is low. T1 acts as  an inverter, and D2 is thus forward biased.  R8 and R4 are therefore effectively in parallel, with the result that IC2 produces a low audible tone. The value of R4 is considerably  greater than that of R8, and so the frequency  of the buzz generated by IC2 is chiefly deter-mined by the value of R8.

When the monostable is triggered, the high  level at the output of IC1 is again inverted  by T1. D2 is reverse biased and so R8 is effectively removed from the circuit. The frequency of IC2 is now largely determined by  the value of R4. The ratio of R4 to R5 and the  value of C4 affect the mark and space periods for the multivibrator: for a satisfactory  ticking sound short pulses with long gaps  between work well. 

Whether a sound is produced also depends  on the voltage on pin 4 of IC2. When the 9 V  supply is connected the monostable is initially inactive and there is no voltage across  C1. Pin 4 (reset) on IC2 is thus low and no tone  is produced. IC1 is activated by a brief press of  S1, which generates a low-going trigger signal  on pin 2 to start the game. C1 now charges via  D1 and IC2 is allowed to oscillate, generating  the ticking sound. 

The pulse width of the monostable sets the  game duration, and can be adjusted using  P1. If the allowed time expires, or if the reset  input to IC1 is taken low (which happens when the hoop touches the wire), the monostable  returns to the quiescent state. This causes IC2  to generate the low buzz sound. D1 is now  reverse biased and C1 discharges through the  relatively high-valued resistor R9. After a few  seconds the voltage across C1 falls sufficiently  that the buzz stops and the circuit is ready for  the next player. 

The circuit can be built first on a breadboard,  so that the component values can easily be changed to suit particular preferences for  game duration and buzz pitch. When suitable  values have been selected the circuit can be  built more permanently on a printed circuit  board. The author used a sheet of plywood  to form a base for the game, the twisted wire  being fixed to the board and wired to the electronics mounted below it. 

Author: Andreas Binner

Railway Points Sequencer Circuit Diagram

Dedicated model rail enthusiasts using sophisticated train and points controllers often have the problem that as their layouts get bigger and more complex, the transformer supplying power to the points does not have enough current to switch several points at the same time. The actuators in the points are designed for ac operation so it doesn’t help by rectifying the supply and adding reservoir capacitors, the coils can overheat and burn out if they get jammed during their travel (ac operation actually helps to overcome friction in the mechanism). The circuit shown here solves this problem by using a sequencer to ensure than only one points actuator can be active at any point in time. During operation the controller will switch all the points on one line at the same time as usual, but the other connection to each coil is connected to the sequencer unit. This circuit will only allow current to flow through one coil at a time. 

Railway Points Sequencer Circuit diagram :

Railway Points Sequencer Circuit Diagram

The sequencer circuit consists of a 555 timer configured as an astable multivibrator clocking a 4017 Johnson counter where the ten outputs are used to switch ten triacs in sequence, enough for ten sets of points. P1 alters the oscillator frequency of the 555 timer and can be adjusted so that each time interval of the sequencer is long enough to allow the points to switch. 

The switching time varies depending on the type of points but is typically between 1 s and 1.5 s. Any points that jam during switching give out a characteristic humming noise in time to the switching frequency so it makes them easier to find. The eleventh output of the 4017 can be connected to an LED (together with a series resistor). This will flash to give a visual indication of the sequencers operation. Power for the circuit is provided by 15 V ac from the points transformer. The B80C1500 bridge rectifier (80 Vpiv, 1.5 A) and regulator IC1 produce a stabilised 12 V for the circuit. Current consumption is only a few milliamps.

Tiny Door Guard Alarm Circuit Diagram

If some intruder tries to open the door of your house, this circuit sounds an alarm to alert you against the attempted intrusion. The circuit (Fig. 1) uses readily available, low-cost components. For compactness, an alkaline 12V battery is used for powering the unit. Input DC supply is further regulated to a steady DC voltage of 5V by 3-pin regulator IC 7805 (IC2).

Fig. 1: Circuit of the door guard

Assemble the unit on a general-purpose PCB as shown in Fig. 4 and mount the same on the door as shown in Fig. 3. Now mount a piece of mirror on the door frame such that it is exactly aligned with the unit. Pin configurations of IC UM3561 and transistors 2N5777 and BC547 are shown in Fig. 2. 

Fig. 2: Pin configurations of UM3561 and transistors 2N5777 and BC547

Initially, when the door is closed, the infrared (IR) beam transmitted by IR LED1 is reflected (by the mirror) back to phototransistor 2N5777 (T1). The IR beam falling on phototransistor T1 reverse biases npn transistor T2 and IC1 does not get positive supply at its pin 5. As a result, no tone is produced at its output pin 3 and the loudspeaker remains silent. Resistor R1 limits the operating current for the IR LED.

When the door isopened, the absence of IR rays at phototransistor T1 forward biases npn transistor T2, which provides supply to  positiveIC1. Now 3-sirensound generator IC UM3561 (IC1) gets power via resistor R5. The output of IC1 at pin 3 is amplified by Darlington-pair transistors T3 and T4 to produce the alert tone via the loudspeaker. 

Fig. 3: Back view of the door assembly

Rotary switch S2 is used to select the three preprogrammed tones of IC1. IC1 produces fire engine, police and ambulance siren sounds when its pin 6 is connected to point F, P or A, respectively.

Fig. 4: Suggested enclosure with major components layout

Author : T.K. Hareendran - Copyright : EFY

12V 3A Power Supply Circuit Diagram

This circuit provides a 12V regu-lated power supply with output current up to 3 amperes. It is spe-cially designed for use with 2m handheld rigs with linear power amplifier and CB portable QRP rigs. The circuit uses monolithic IC CA3085 voltage regulator in 8-lead TO-5 package.  Its salient features include good load and line regulation, output current up to 100 mA (which can be increased to several amperes with additional pass transistors), output short-circuit protection, and lower input voltage.  A low power dissipation is achieved by driving external series-pass transistor 2N4241 (T1) from  pin 2 of CA3085.

Circuit diagram :

12V, 3A Power Supply Circuit Diagram

Normal output pin 8 is returned to ground via diodes D3 and D4 to ensure error am-plification operation in the linear region. Ripple rejection is approximately 50 dB on no load and 35 dB on full load.  A 2x2x2.5cm aluminium heat sink fas-tened onto a 1.5mm blackened aluminium sheet of 12.5cm2 area on 2N4241 helps the circuit in dissipating heat without ex-ceeding maximum device ratings. CA3085 can dissipate up to 650mW power in free air, without any heat sink.  AFCO-make C-05-4 heat sink is suitable for this IC. An improper heat sink may cause device junction temperature to ex-ceed the limit, resulting in progressive deterioration of the device.