Editor’s note: This design idea offers a solution to the common issue of a TV automatically restarting after a power outage. Oftentimes, power may be restored when the consumer is not present and unknowingly left running. This could be due to several reasons, including the HDMI-CEC settings on the TV or simply an automatic restore factory setting. While it a useful setting to have enabled, it would be helpful to ensure the TV will not be automatically turned on when power is restored after a power outage.

Introduction

Present day TV designers take ample care in power supply design such that TV comes “ON” automatically after a power shut down and resumption, if TV was “ON” before power shut down. If the TV was “OFF” before power shut down, it continues to be “OFF”, even after power resumption. This is an excellent feature; one can continue to watch TV after a brief shut down and resumption without any manual intervention.

At times, this can lead to certain inconveniences in case of long power shutdowns. The last time this happened to us, we were watching TV, and the power suddenly went off. At some point during this power outage, we had to leave and came back home after two days. The power may have resumed a few hours after we left. However, as per its design, the TV turned “ON” automatically and was “ON” for two days. This caused discomfort to neighbors until we returned and switched the TV off. What a disturbance to others!

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TV power-ON aid

I designed the “TV Power-ON aid” gadget in Figure 1 to overcome this problem. Mains power is fed to this gadget. Power is fed to the TV through this gadget. Once the SW1 switch/button is pushed, the TV receives power, as long as mains power is there. If power goes “OFF” and resumes within say, a half hour, the TV will receive power from the mains without any manual intervention, like the original design. If the power resumes after a half hour, where it is likely you may not be near the TV at that time, the power will not be extended to TV automatically. Instead, you will have to push the button SW1 once to feed power to TV. This gadget saves us from causing discomfort to the neighbors from an unattended TV blasting shows: a problem anybody can face during a long power outage when he/she was not present in the house.

Figure 1 TV power-ON aid circuit. Connect mains power to J1. Take power to TV from J2. Connect power supply pins of U2, U3, and U4 to V1. The grounds of these ICs must be connected to the minus side of the battery. These connections are not shown in the image.

Circuit description

The first time, you will have to press momentary push button SW1 once. Relay RL2 gets energized and its “NO” contact closes, shorting SW1. Hence, the relay continues to be “ON” and power is extended to TV.

When mains power goes off, RL2 relay de-energizes. Through the “NC” contact of relay RL2, the battery (3X 1.5 V alkaline batteries) become connected to the OFF-delay timer circuit formed by U1(555), U2 (4011), U3 (4020), and U4(4017). As soon as the battery gets connected, this circuit switches “ON” the relay RL1 through MOSFET Q1 (IRLZ44N). Its “NO” contact closes and shorts SW1.

The timer circuit holds this relay for approximately a half hour. (The time can be adjusted by suitable selection of C2). If power resumes in this half an hour period, since SW1 is shorted by RL1contact, the power gets fed to TV automatically. If the power resumes after a half hour, since RL1 gets de-energized due to the OFF-delay timer action, its contact which is connected across SW1, is opened and power is not extended to TV. This is a safe condition. When you come back, you can push the button SW1 to feed power to TV. The RL1 coil voltage is 5 V and the voltage of RL2 is either 230 V AC or 110 V as needed.

The U1 circuit works as an oscillator. The U3 circuit works as frequency divider. This frequency is counted by U4 circuit. When time delay reaches around 30 minutes, its Q9 goes high. Hence U2C output goes “LO” and RL1 gets de-energized. Whenever power goes off, timer circuit gets battery voltage through “NC” contact of RL2. When power resumes, battery is disconnected from timer circuit, thus saving battery power. A Lithium-ion battery and charger circuit can be added in place of alkaline batteries, if desired.

Jayapal Ramalingam has over three decades of experience in designing electronics systems for power & process industries and is presently a freelance automation consultant.

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